This is how stomach capacity used to work in one of the the earlier versions. Your character could also lose weight if not eating for a while. I removed both mechanics because I felt it didn't really fit with how I wanted the game to be played.
When designing the game, I'm trying to have a rewards-first approach, I privilege encouraging behaviors I want the player to have rather than punishing the ones I don't. And since this is an inflation/fat fetish game, the most basic rewards I can give the player for playing the game are weight gain and expanded stomach capacity. Which would make reducing the character's weight and stomach capacity a punishment.
Debucon is a "long-term" kind of game. You don't just binge play the game in one afternoon, it's a game you come back to every now and then. I'm not expecting the players to launch the game every single day, but rather play it for a bit, forget about it for a few days (or weeks), then boot it again to get a quick fix, and so on. I don't want the player to feel like the game is a daily chore, and I don't want them to feel penalized for taking a break. If I reduced the character's stats because the player didn't feed it for a few days, I would be penalizing a behavior I don't even actually want to discourage in the first place. Which is why I decided against decreasing weight and stomach capacity over time, if the character's stats were to be decreased, it would have to be a choice deliberately made by the player. Sometimes you need to sacrifice realism to improve other parts of the experience you want to convey.
Sorry for the long rambling, I like discussing game design (and I'm also open to hearing arguments against my stance!)