With more knowledge about the anatomy of the horse being available, this is now known to not be true. Draft horses can pull a lot more weight proportionally to hot blooded horses, but can only safely hold a little more weight on their backs. They were bred to pull, not suffer under the weight of fats. Not that destriers really exist anymore, but even they were bred to carry a lot of weight on their backs, knights in full armor, totaling a whopping 200lbs.