It depends on the draft. My boy gets FAT, as in, literal rolls, if you give him 1 cup of feed and 3 flakes of hay (twice a day), while my mother's smaller horse gets skinny if you feed him less than 2 cups + 5 flakes (twice a day).
On a diet, my draft gets 1/2 a cup twice a day, and one flake of hay, and I still have to work him hard to get weight off of him.
It really depends on the horse, but I hear my boy's metabolism is pretty common from other draft-owners I know. My friend's draft is on grazed-down pasture only, and down right fat, while his other horses are a bit skinny.