Premise:
Two snotty thin sorority girls constantly poke fun out of fat people. They get invited to a party by a friend of a friend, only to end there seduced by respectively: hunky guy and a somewhat plump girl. As they start playing in bed, both girls start inexplicably gaining weight, which disgust them, but in the same time they are aroused by the ongoing intercourse. Eventually then end up being two fat chicks, disgusted with themselves, but very much enjoying the difference in bed all that fat gives, and try to move on with their lives. So I guess that counts as a bitter-sweet ending?
I know it was for sure on Hentai Foundry and I've already manually searched for it and didn't find it. Can't even recall the title, but it was definitely there, probably taken down by the writer.