We're not necessarily talking about damage to the legs, which anyway is more a question of tendon and ligament damage than bone damage. Most of the issue is back pain, or pain that's referred from the back to the shoulders and hindquarters. I don't think the height or weight of the horse are a good guide in any except the most general sense; conformation is essential in considering appropriate size. (Ponies, for example, are structurally stronger than horses and can carry a larger percent of their body weight without discomfort.)
Please note that the message to which I replied was devoted specifically to the size of the cannon vs the rider’s weight.
As for back pain, your own post settles that. As you said, little horses can usually carry a higher percentage of their weight.
Size does not matter. The individual horse and the saddle fit, pad selection and rider’s melding with the motion of the horse are much more important.