Dunno, just spitballing. I don’t know your skill level and I don’t really know what your goals are with this game, so I’m practically just guessing randomly, ha.
True, without knowing what’s a reasonable scope for your skill level + your level of commitment + whether you’d like to monetize it or not, we are just spitballing.
OP: Reflecting on it, waiting a year for significant progress is very long. Even in real life, significant weight gain can happen within a few months. You might want to adjust your timeframes down for casual audiences. Maybe they’d like to see results within a week or a few months. Maybe that infringes on your original idea, but you could easily give the player a few options during app initialization that decide weight gain rate (Very Fast, Fast, Realistic, Slow, etc.). Or, maybe this is reflected in a Resistance trait, where the player’s character might resist eating too much before their self-control is brought down.
I agree stuffing mechanics would be a good immediate reward. A nutrition system adds more game elements as well, although you’ll probably be doing a lot of research and adjusting values back and forth trying to get that game element to “feel right” if that’s within your scope.
I like the idea that feeding too much of one food has some sort of effect or counter, like a health system could work - or you could implement it in an unrealistic, gamified way like I’ve seen some other games here do. For example, excess dairy leads to breast weight gain, excess fat leads to lower body weight gain, and carbs lead to belly weight gain, although that might also infringe on your realism focus. After all, in these sorts of games, you have to use your resource management skills to balance what you need with what you can afford (i.e. in Indulgences, you balance Affection, Income, Stomach Capacity, each character’s Stamina, and Time). If it’s not designed with all use cases in mind, though, players will easily be able to find the path of least resistance to make the fattest character.