From what I heard, a horse gets a hay belly because they over eat hay that is lacking enough nutrients. They eat more hay just to get enough of the nutrients. You could try giving a mineral block(different than a salt block), or feeding a higher protein suppliment like beet pulp. One of ours was so bad he looked like he was pregnant with twins. We started him on beet pulp about 4 times a week, and it went away. I've also heard someone say to give some probiotics? I think that's something to help them digest better. But I'm not sure on that.