I find that same thing happens to me when I eat a lot of really processed food.
I've heard that the big tummy happens because your intestines are inflamed trying to digest too much stuff that isn't readily digestible (breads, sweets, etc), kind of like a hay belly on a horse (to make it sound terrible! Sorry! Haha)...but it could be not eating a balanced enough diet just as much, I suppose!
In any case, what works for me is trying to eat things that are as unprocessed as possible. I make a lot of my own food vs buying ready made preservative laden store food, etc. I eat a lot of salads with meat, that I bought from the store and cooked, on top...that sort of thing. Pretty much, if there's an ingredient on the label that I can't look at and say "That is basically ______, that comes from a ______ plant/tree/bush" then I try not to eat whatever it is.
I'm not super crazy strict about it but I find that my tummy stays smaller and I feel better if I eat like that.
I've also read that doing exercises to strengthen the ab muscles can help but since I'm completely unmotivated to do exercises, I don't have any proof.
Hopefully that was mildly helpful, I realized that I don't have any real advise about balancing your diet...