I agree with those who say to feed more.
My tb got about 19 pounds of food a day, he is finally to the point where I could cut down. When I got him he was really skinny, he had only been used for trail riding and was on a hay diet. I've been feeding him sweet feed, senior feed, whole oats, alfalfa/bermuda pellets (full scoop each) and 2 flakes of alfalfa a day (and a bunch of supplements).
I also ride the s..t out of him. This is what he looks like now: http://www.flickr.com/photos/8309094@N03/2698022578/