Tall does not necessarily = good for weight bearing. It is a common misconception that full drafts are always a good choice for a heavier rider. It does not matter if you ride a 15hh or an 18hh horse as long as they have good bone, a short back and a strong loin coupling.
Traditionally, here in NA, pure drafts are bred quite long and lean if they are "hitch" or show bred - you might find working bred drafts to be a bit more suitable. Mine is a (what we presume) percheron cross of some type but she has such a short back that a western saddle to fit her that doesn't interfere with her hips is difficult to find!