Hi there, I just wanted to add a couple more tips that might help. And thanks Jamie for the ADM information! We can't get it here yet.
First there is a more accurate calculation table for miniature horse weight, actually three, developed by the Kentucky Equine Research Center, here's a link;
have tried all three on one horse and they are very accurate and helped me a lot when I had the very same problem as you with a 2 year old rescue filly. Only she was very small, 30.25 inches and always looked like a water balloon. So I was in the same situation tho she was a little younger. I started her back on a little alfalfa, slow feeder hay nets, and now use Safechoice Special Care which has no corn, and lower starch and fat than the original. I did all the things advised here and yes unlike my Quarter Horses, their mouths never stop searching. Here is what helped her,
Besides the added alfalfa, I started using Ration Plus after trying everything else including probiotic paste. It is a supplement for the fermentation bacteria in their hind gut, and I started using Karbo Combo, which is an immune booster and more. Then she also turned three. The result was she lost the bloated belly, quit eating everything in sight including manure, and they all started looking better.
I have a two year old now again and the same problem. Everything is the same as you again, the same feeds, same bloated horse eating everything constantly. I have tweeked her schedule and routine a little and tomorrow will start her on the Ration Plus. It sure helps them digest better so that they lose that bloated belly. Wish it worked on humans. I also have a lot of muzzles.
Your mare is a pretty girl, hope you can give us updates, and I agree that she doesn't look obese at all. Now mine do but they are much smaller.