Glitter Jelly:
I've often wondered about that, since this theme is so prevalent in the stories section...
Letters And Numbers:
If I had to guess, there might be 2 things going on:
1) That kind of revenge-by-getting-fat trope is common in a lot of media, it's been around forever, and I'm sure there are quite a few people in this community today who discovered they liked weight gain (or erotic weight gain media) because of tropes like that in "straight" media -- they just liked it for a different reason than how the author intended, maybe. Maybe not.
2) A lot of the stories on this site are taking well worn tropes like this one and putting the author's own take on it, and the author might not be thinking that hard about the origin of the trope they're mimicking, because it's something they always thought was hot. They might not have seen it the way it was intended to be seen, kinda subverting a problematic trope, but maybe inexpertly making that clear. Or not. It's amateur art, though, people trying things out.
Munchies:
I think this is true in terms of surface level, individual reasons. But the popularity also speaks to feedist culture. As much as feedism is countercultural, feedist still live in a fatphobic world. I do not think a lot of feedists have interrogated these influences as a whole.
I noticed revenge weight gain stories follow a general beat. Mean person is skinny and hot. As punishment for their sins, they get fat. Society rejects them, their life is ruined, and then they get fatter. Bonus points if the mean person joins the other loser fatties or the loser fatty they bullied become skinny and hot.
Notice how this frames fat as bad and skinny as good. There are, of course, variations on an individual level, but this is how the bulk of them go. From where I sit, this suggests feeling ashamed of their kinks and fetishes. They find fat sexy (the porn part) while also negociating some level of fatphobia (the revenge part).
But I suppose it makes sense. A lot of people sexualize what they find taboo.
Makes a lot of sense.
I'm not sure how I feel about these stories... I've read many of them but I often feel bad for the protagonist as I'm not into gaining weight myself and feel strongly that secretly fattening someone is not OK.
I've been bullied a lot for being heavier than my classmates so I'd be lying if I pretended I wouldn't feel some satisfaction if I came across one of these mean girls and she had gained a lot of weight.
I used to think of it as karma but I acknowledge this is the socially conditioned fat-phobic part of me that is responsible for that kind of thinking.
Even if they were much fatter than me, I wouldn't go as far as pointing it out to them or humiliating them, however. I wouldn't want them to feel the same way I felt because it leaves scars on your psyche.
I think intentionally getting fat as a way of saying f**k you to diet culture and revolt against current beauty standards. I'm not there yet but can see myself getting here if/when I'll be fed up with trying to shrink my body against its nature.