Project F.A.T. is one notable thing meeting your criteria, if you haven’t heard of it already.
Superfatty RPG Expanded is another famous one, though one that must be paid for. The old version was free though. Supernatural/afterlife themed
Vale City is another famous one, in a modern setting.
Total Re-Fat is post apocalyptic sci-fi.
A Huge Hallow’s Eve is a monster collector where your monsters level up through various transformations, most of which involve gaining weight. As of right now however only a random dungeon generator/battle system demonstration exists.
Fatty Text Adventure Game is adjacent- you burn fat for increased abilities. and it has a belly capacity stat that allows you to eat more by eating past your maximum.
In The Fat Tournament, you don’t control the fighters themselves but instead it’s a management sim game where you try to get them as big and skilled as possible for computer ran fights.
Food Frontline is a tactical game with some mobile-game-like elements.
In Pandara’s Sphere, you don’t gain levels from wg directly- although many ability boosting foods also boost weight, and weight is a key variable in powering several techniques for the main character.