She is on a dry lot (5ac) and gets two - three flakes of orchard grass hay and 1lb standlee alfalfa grass pellets as a carrier for vitamins (mvp). I test my hay every year and it's low sugar <10%.

I ride 3 x week for about 1-2 hrs. And she always looks better after riding but if I skip a week she is back to a hay belly again. It's so bizarre. She gets dewormed and teeth done, chiro, psyllium pellets. I have had lots of horses and never had one this bad. She needs to lose probably 100lbs. I may change her vitamins hoping that will help. Is there a supplement that helps or what can I do? 

Interested in help, also.  I have the same problem with my gelding.  Always has a belly on him.  Had to remove my back cinch because it wouldn't buckle!  

Ride more, less food. Not unlike people, intake exceeds output equals belly.