Join Date: Feb 2012
Location: Alberta, Canada
I would hesitate to put more than 30%(rider and tack) on any horse, with a few exceptions.
Conformation and fitness are huge. A long backed 1200 horse will likely be less capable than a short backed 800lb horse. A horse with conformation issues may need a lighter rider.
A less balanced rider will be harder on their horse than a balanced one.
Also what is being done with the horse is factored in. Jumping is hard on a horse, add a very large rider and you are certainly putting unnecessary strain on joints.
Generally, if the horse is decently put together and not being ridden too hard, a larger rider should be fine. I have seen horses struggling under riders that weighed too much for them, and it makes me cringe.
Keep in mind that the small horses bred hundreds of years ago that carried large amounts of weight had to be sound. An unsound horse, unless it could be repurposed as a breeding animal, was destroyed. They were all bred for soundness, unlike many types of horses today where the focus is on one specific thing(like speed) or on looks, and where a huge percentage of horses have regular lameness issues. The horses today are typically not as tough, sound, or as well put together from a using standpoint.