Clothing malfunctions are turn-ons for obvious reasons - popped buttons, rips and tears, etc - and it’s what I voted for. It really puts on a display of how greedy the wearer is, so ravenous that they’re eating so much so fast their clothing can’t even keep up as it eventually surrenders and makes way for the pounds and pounds of fat that are stressing it. The character’s reaction also has its own appeal, either humilated and embarrassed at themselves but unable to stop, or unphased and apathetic, so lost in their haze of gluttony to even notice or care that they’ve eaten themselves to the size of a blob with their entire outfit in tatters around them.
On the other hand, I feel like clothing that still fits (albeit possibly poorly) also has appeal in its own way. Oversized shirts that leave a little bit of belly uncovered, stretchy pants pushed to their limit as they attempt to contain a huge gut, things like that. Also, a character having a fitting wardrobe sort of implies that they’re used to being at their size for a time – having them in a sized-up version of their usual outfit that can contain their new bulk kind of normalizes it, I guess. Like, if a specific character is drawn fat, but in their normal outfit (even if it’s a bit of a poor fit), I kind of just imagine them doing the things that they normally do, except with a few hundred more pounds, which is a turn-in in its own way. Maybe I’m just being weirdly specific and this is just me.
Although, this is just kind of in general – both of these are good, but if it’s specifically about quick weight gain, I prefer damaged clothing.