For a heavier rider look for a horse with a nice short back, and a nice short loin... Put it with a deep loin girth and well sprung ribs. Make sure the back blends smoothly front and back.
If you think of the horse's back as a suspension bridge, the wither and the LS joint are the anchors. Without strong anchors the bridge will sag and breakdown over time. (For those wondering, the LS joint is one of the critical points that not only offers an anchor point for the back, but it also controls how well the horse can "shift gears" and bring their hindlegs under and through from behind - which is what allows the horse to lift it's back while working. Ideally the LS should be placed at or ahead of the point of hip, this gelding looks like it falls behind that point, reducing the strength)