Maybe it's cuz I got that service dog in me, but I love fantasy scenarios where belly expansion happens from fulfilling some kind of job. For example, being a creature that can safely eat and digest literal garbage. It's better to feed it to you than to have it rotting and harming the environment in some landfill, right? So the creature eats so much garbage that it gains a lot of weight. It could be a creature with a huge stomach and a large mouth that can fit an entire garbage bag inside. Or what about a creature that delivers things, and stores them in their stomach for safety? Or getting inflated in public for everyone's cheering entertainment? Maybe it's not even a fantasy thing, and instead it's something like being a taste tester, and the food is so fattening and you're such a hard worker...