You are incorrect to say it is not a factor of weight, but that is not the only factor, which I alluded to when I said "though not completely." Even for 1,000 lbs thoroughbreds weight matters. Jockeys are small and they have handicapped horse races where they add weight to horse to slow it down. Weight absolutely matters. You even said: "...most animals can't carry any significant weight on their back,,," The question is what is a significant weight? This is relative to each animal, so I can't really answer that. However, I bet the # goes up with training and selective breeding of the type you would expect to find in animals raised to be ridden.