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.

I wouldn't drop her feed.  Your feeding very little now.  Start with putting her on Thyroid.  Discuss dose with your vet, but usually 1 scoop per day is enough.

Thanks for the replies! Which supplement do you think I should try? 
I know less work- less food but what normally works for horses- isn't working on this mare. 2-4 flakes has always been fine for my horses. So frustrating! She's a nice mare. Thickkk. I tried probiotics too to see if that would shrink that belly but not so far .

This is exactly what was going on with the gelding that I put his feed in a slow feeder net, he was heavy and kind of had gotten a cresty neck ( I ride 5 + days a week so knew it wasn't lack of riding) He would inhale his feed , I llike keeping forage in front of them pretty much 24/7 and sometimes the pasture gets too short to provide much, then he would get another flake of hay and it was just a vicious cycle of him being chubby and putting feed in front of him. The hay net hellped that and then I got the thyroid support from Kristen Holt and ran 2 or 3 bags of it thru him.  Huge difference.