Pablo Sandoval stepped off the double-decker bus that carried him along the jam-packed streets of downtown San Francisco and walked into a staging area in City Hall. Two nights earlier, halfway across the country, he scored the go-ahead run and caught the final out of the deciding game of the World Series. But now, inquiring minds wanted to know whether he would agree to a new contract with the Giants, who signed him as a 16-year-old in 2003. “I want to wear that jersey the rest of my career,” Sandoval said.

Why not? The portly switch-hitter with the bright smile and the cool nickname—“Kung Fu Panda,” coined by former teammate Barry Zito as a tribute to Sandoval’s personality and girth—was immensely popular among baseball fans in the Bay Area, even inspiring a line of furry orange-and-black panda heads. A two-time All-Star, he had just won his third World Series in five years. And the Giants wanted to keep him in San Francisco for a long time. Long enough that they were preparing an offer that would top any he received on the open market.

So when Sandoval sat alongside victorious teammates and club officials at a press conference and declared that he wanted to be a Giants player for life, there wasn’t any reason to doubt his sincerity.

Except it wasn’t true.

Everyone is entitled to a change of heart, of course. And maybe Sandoval never had as much fun as it appeared in San Francisco, far from his family on the north coast of Venezuela. But it’s still a mystery to Giants assistant general manager and Framingham native Bobby Evans why Sandoval chose to cut off negotiations in the second -to- last week of November, before San Francisco could make its final bid. The Giants were willing to go even higher than the winning five-year $95 million offer that makes the 28-year-old Venezuelan the game’s second-highest paid third baseman, behind David Wright of the New York Mets.

“We weren’t going at it halfway,” Evans says. “He was a Giant, and for us, he should’ve always stayed a Giant. I think there’s no player that loved his fan base as passionately or had a fan base that supported him as passionately. It was tough to see him go.”

Turns out, it wasn’t as difficult for Sandoval. Less than a month later, he was standing at Fenway Park, his arm around a man in a panda costume, being introduced as the new third baseman for the Red Sox.

Having completed his spring-training workout shortly before noon on a blazing hot February day in southwest Florida, Sandoval returns to his locker at the Red Sox’s player-development complex and fields my first question—an obvious one. Why leave the only team you had ever played for, to say nothing of a desirable city where you were always cheered, after winning a third World Series ring, especially since that team wanted you back and was willing to pay top dollar to keep you?

“I did a lot of things out there. I wanted to try something new,” Sandoval says. “I told myself that’s what I wanted—a new challenge.”

Yeah, his friends aren’t buying it either.

“They thought I was crazy that I left the only team I was playing for to come here,” Sandoval says, a mischievous smile stretching across his face. “My friend told me it like this: I come from first place [San Francisco] to last place [Boston]. Crazy.”

A few weeks later, the truth comes out. Sandoval tells Bleacher Report that he felt the Giants disrespected him before last season by making an offer that Evans claims to have been for four years and $85 million—or one year and $5 million less than the extension they gave to right fielder Hunter Pence in 2013. Then, Sandoval delivers the dagger to the Giants’ backs. He says he won’t miss anyone in the organization aside from Pence and manager Bruce Bochy.

What ensues is a long-distance volley of insults between Sandoval and current and former teammates. On Facebook, ex-Giants infielder Aubrey Huff calls out Sandoval for being selfish, to which Sandoval says in a Spanish-language interview with ESPN Deportes, “Who’s Aubrey Huff?” Meanwhile, Giants outfielder Gregor Blanco, one of Sandoval’s closer friends, tells the San Francisco Chronicle that the Panda will come to regret his disparaging comments. But regardless of the nasty back-and-forth, by the end of spring training, one thing is clear: Sandoval is much less concerned about burning bridges with his old team than building a foundation with his new one.

“When I got so many [free agent] opportunities on the table, I said I was just going to take my chance and go with a team that I feel comfortable with,” Sandoval says. “The opportunity that these guys showed me, I love being part of the Red Sox. So that’s the chance I took.”

It’s true: The Red Sox did make Sandoval feel wanted. Then again, what choice did they have? During the past two seasons, few teams derived less offense from the third base position. Rather than fulfilling his vast potential, injury-prone Will Middlebrooks began getting more attention for dating former NESN reporter Jenny Dell. Tired of waiting for Middlebrooks, the Sox went all-in on Sandoval, a significant upgrade even though his OPS (on-base percentage plus slugging percentage) had steadily declined from .909 in 2011 to .739 last year.

Sandoval had never been to Boston, so in mid-November, as part of the free-agency courtship, the Red Sox invited him to town for a red-carpet tour of Fenway Park. He was accompanied by his older brother Michael, who’d signed with the Minnesota Twins in 1999 and advanced to Class A in 2004 before his career fizzled. He’d been an early influence on Pablo, who had an interest in basketball before gravitating to his brother’s favorite game.

“I didn’t know baseball a lot, so he showed me a lot of things about it,” Pablo says, recalling how Michael would pitch to him when they were kids. “We played together back in the yard. It’s big time for me that he’s sharing this with me, because he cared about me when I was a little kid. This means a lot.”

The brothers were ambushed at Logan Airport by a Ch. 7 (WHDH-TV) news crew, so there weren’t any subsequent Panda sightings around town. Despite reports at the time that he went to dinner with David Ortiz, Sandoval says he intentionally stayed in his hotel when he wasn’t meeting with Sox officials on Yawkey Way.

“I didn’t want no one seeing me out there,” says Sandoval, who was also being wooed by the Giants and the San Diego Padres at the time. “I hope that during the season I’ll have a chance to see the city and eat at a couple restaurants. But I didn’t want no one seeing me at that time.”

Regardless of whether they broke bread together, you’d better believe Ortiz’s presence helped draw Sandoval to Boston. Panda has long admired Big Papi, and the chance to play with a slugger he respects was enticing. During his introductory press conference at Fenway Park on Nov. 25, Sandoval even dubbed himself, newly signed left fielder Hanley Ramirez and Ortiz as “Tres Amigos”—a dream come true for every Red Sox marketing executive.

“He’s a great player, one of the best in the league,” Sandoval says of Ortiz. “I just want to spend time with him and have fun.”

Apparently, it wasn’t always fun in San Francisco.

Although Sandoval finished second to new Red Sox teammate Ramirez in the National League batting race in 2009, he weighed more than 270 pounds. Giants officials said they feared that if they didn’t intervene, he would eat his way out of the big leagues.

So they designed an offseason training program—“Operation Panda,” they called it—that introduced Sandoval to weight lifting and changed his diet. He lost about 30 pounds, although both he and the team were careful to keep his actual weight a secret.

Former Giants center fielder Aaron Rowand recalls that it “wasn’t a pleasant situation for Pablo because they put a lot of pressure on him to go and do that.”

“It was different,” Sandoval says. “They made me do some different things I didn’t always like. But you learn a lot of things from that. After that, you keep your own things. You hire your own trainer and find things you like about it.”

According to Evans, Sandoval’s weight was “a constant discussion point” during his seven seasons with the Giants. “We’ve always felt that there wasn’t a lack of work ethic on his part, but there was clearly more at play in terms of maintaining a weight that we thought was OK,” Evans says. “It was a challenge. But I think he’s a special individual, a very talented ballplayer, and unfortunately he has that challenge. He works hard to get it to the right level, but it’s just hard to maintain. There’s no other way to say it.”

But regardless of his fluctuating waistline, Sandoval produced. He came up as a catcher, made the full-time switch to third base in 2010 and has been a borderline Gold Glover ever since. At the plate, Sandoval is what Rowand calls “one of the best bad-ball hitters I’ve ever seen.” There seemingly isn’t a part of the strike zone that the Panda can’t reach. According to numbers compiled at Fangraphs.com, he has swung at 58.3 percent of pitches during his seven-year career, including 45.7 percent of pitches that are out of the strike zone, and has never struck out more than 85 times in a season. By comparison, Ortiz has offered at 44.8 percent of pitches, 22.2 percent out of the strike zone.

Sandoval doesn’t put on power displays in batting practice. Instead, he sprays line drives all over the field from both sides of the plate. It’s that ability to make consistent contact with most pitches and drive the ball into the power alleys that led the Sox to identify him as a target during the offseason.

“It doesn’t matter where the ball is thrown. This guy is going to square it up and hit it really hard no matter where you throw it,” Rowand says. “I mean, he can just flat-out hit. There’s no rhyme or reason for it because some of the things he does people would call a little unorthodox, with just swinging out of his shoes every pitch. But he got it done. He has that knack, that hand-eye coordination.”

Rowand played with Sandoval from 2008- to 2011, during which time Panda batted .307 with 64 homers and an .857 OPS in 463 games. But his real breakthrough came during the 2012 World Series. He hit three home runs in Game 1 against the Detroit Tigers and batted .500 (8 for 16) overall in the Giants’ four-game sweep.

Sandoval’s playoff performances are downright Ortizian. Last year, he set a postseason record with 26 hits, and if not for the pitching heroics of Giants lefty Madison Bumgarner, he would’ve collected another World Series MVP award to put next to his 2012 trophy.

Like everyone else, though, Rowand once wondered how long Sandoval could stay in the big leagues if he kept carrying so much weight. Even now, he questions whether Sandoval will have to move to first base or even designated hitter before his Red Sox contract expires.

But the Sox knew they needed to order an XXL uniform when they signed Sandoval. So when a Boston.com producer snapped an unflattering photo of him with his belly hanging over his shorts and posted it above a snarky comment on Twitter, club officials barely blinked. As general manager Ben Cherington says: “Pablo is always going to look like Pablo. You’re going to notice him when he walks on the field.”

In fairness, Sandoval’s weight didn’t scare the Giants out of making a long-term commitment. Evans says Sandoval’s age relative to most free agents helped put them at ease. But it’s entirely possible that so many years of hearing about how much he tips the scale contributed to Sandoval’s desire to make a change.

Not even the spring training photo flap could wipe the smile off Sandoval’s face. He re-enacted the picture by sticking out his belly for what teammates called a “Panda Pose” in a self-effacing Twitter picture with Ramirez and pitcher Joe Kelly. He even challenged his critics to watch him work out.

“Everybody’s posting pictures of me. I don’t care,” Sandoval says. “I love them. I’m making fun of it. I’ve put other pictures out there making fun of it because I want [critics] to spend one day with me to see how hard I work.”

The Panda is getting feisty. And he’s having fun again. Maybe he really did need a change after all.

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Scott Lauber covers the Red Sox for The Boston Herald.