Is she a Haflinger/ they are draft horses and while you do not want them to be obese, they have a very well sprung rib cage (in general) and this contributes to them appearing fat when they are not.
She is also in winter coat so to judge is a bit difficult.
When you run your hand over her side, can you feel ribs? If not she can go down a bit more in weight. If you can, then she is just right.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer