I'm not sure specifically what the 'big belly hold story' is as it could be something very unique or personal. It might be a story related to someone with a big belly and an interesting event or situation they were in.
In an erotic story, there's no fixed rule for how much a belly can hold. It's all up to the author's imagination and the themes they want to explore. It could be a small amount for a subtle effect or a large amount for a more dramatic scene.
It really depends on an individual's tolerance and interest. Some people might have a high capacity to handle such content, while others might find it overwhelming quite quickly.
One cause could be emotional eating. People might overeat when they are stressed, sad or bored. Another cause can be a lack of portion control. If one constantly eats large amounts of food without considering the right portion sizes, it can lead to overeating and eventually a big belly.
Exercise regularly. Cardio exercises like running, swimming or cycling can burn calories and help reduce belly fat. Also, strength training can increase muscle mass, which boosts metabolism.