FWIW: I rode Cowboy when he was both pretty fit and I was a bit heavier...about 33% of his body weight. He showed no signs of soreness but it was apparent he had more difficulty balancing my weight in a sharp turn - which his 13 hand, short wheel base model was very capable of providing. I felt OK with riding him as long as the rides were mostly straight lines. I wouldn't feel good about doing a lot of turning on him.
But then, turns tend to be harder on a horse than straight lines. Collected work tends to be harder on a horse than less collected. Speed work is harder than slower riding. That is where the variables kick in. For normal field work, the cavalry did more straight lines than sharp turns, and a horse carrying 250 lbs wasn't expected to go fast, turn hard or stop in a slide.
Someone who rides like I enjoy - mostly straight lines, usually at a walk or trot, with the horse welcome to slow if the footing or exhaustion (lack of water here in AZ) requires it - will have a different limit than someone trying for high level dressage, barrel racing, reining, jumping, etc.
Like the cavalry, I approve of dismounting periodically and letting the horse rest for 5 minutes, or at least walk with me for 5-10 minutes. If a trail gets too ugly looking, I have no problem with dismounting - easier on the horse and safer for me. Going up hill shifts weight to the rear and raises metabolic requirements, and I normally let my horse walk up hills. Or an easy trot. This is a view I expect to see regularly with Bandit:
5 minutes an hour does wonders for my knees. I suspect it helps him too. BTW - he's a very slender horse.
Trooper was ridden by a heavier guy than me, but he was used for herding sheep. There isn't a lot of sustained running, sliding stops or jumping involved for a horse herding sheep. A horse used for herding sheep, even if it is for 12+ hours, will have a higher weight limit than a horse in competition.
Based on riding my horses at around 22-33% of their weight, I figure someone can do my type of riding up to 30% without me worrying much. After 30%, I think a person would need to be careful. But one cannot separate a weight limit from what you do while riding.
Bandit (I'm guessing 800 lbs tops) has carried heavier guys than me for long distances, but I've noticed he tends to brace his back at a trot. His front feet are still too tender after pulling shoes for me to trot him far, but I'm doing it in two point (yes, entirely possible in a western saddle) and he seems to be relaxing his back more at a trot. I can't blame him. I've carried packs and have braced my own back when the weight gets toward my upper limits. My tack & I are around 200 lbs total, but I need to be conscious of how he carries me and how confident he moves in various paces. Good balance, though, and willing. I think he'll relax when he realizes we aren't going to trot 20 miles non-stop, or with me sitting the trot the whole time. Posting and two point are good things for a western rider to keep in his bag of tricks!
I think a reasonably fit horse can easily carry 25%. Even 30% - if the riding is the type of riding I do. I think that limit might drop below 20% for some types of riding, and there are some stuff done in horse sports that just plain makes me unhappy to watch, regardless of weight.
For the OP, I think it would be entirely possible to ride a pony or horse at 25-30%...just don't expect the pony to max perform all day doing it. If you feel your horse bracing its back or struggling to keep its balance, then back off - either in weight or in what you are asking of him. No hard rules. Just listen to the horse. It sounds like the OP is someone who cares enough to listen...