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.

Put hay in a slow feeder net, I tied mine to the botton of my feeder so that the gelding I had issues with was still eating in a grazing position and couldn't get his shoe stuck in the hay net from pawing.Also might need to run herbs for some thyroid support.

Went through this for a few years with my mare, until we realized she's insulin resistant. She is on Thyro-L to boost metabolism and low sugar everything, including soaking her hay. 10 months later, she looks like an entirely different horse. She literally was on the last hole on the back cinch, now she actually looks like a barrel horse and is clocking! Get sugars checked, it has made a huge difference for my mare. Happy to answer any questions you may have.