Is bigger always better? Depends who you ask. But it’s hard to argue with the (mostly gay) crowd that swells at this Dorchester restaurant on Saturday nights, when the dining tables are pulled aside and the space fills with sweaty lovers of Top 40 remixes. Magnum keeps things interesting with frequent theme parties, from ’90s nights to Speedo soirees to a recent “Anything Goes But Clothes” costume contest. Hey, dbar: We like what you’re packing.