It sounds like she has a thyroid problem. Horses get thyroid problems the same as people do. No matter how little you feed or how much exercise she looks fat. There is a product called Thyrol that you can give her. If I remember right it is one scoop/100 pounds of body weight. I always divided it into 3 feedings mixed with a pound of sweet feed i.e. 3 scoops am, 3 scoops noon, 4 scoops pm for a 1000 pound horse.