Well I am going to throw a spanner to the works and expect to get a whipping.
What makes us think the horse was ment to carry any weight on its back.
I remember giving my kids a horse ride in my back on all fours, kid on back and no saddle to spread the load or soften those bum bones from digging in. Not the best example but I did not last long. Back began to hurt.
The horse was not designed with the object of carring us on its back.
I completey disagree, the horse has specifically been designed to carry a load on his back, through thousands of years of selective breeding, Lots of people have tried to ride other critters, I am sure Grog and Nog climbed on others before they caught a horse, the horse worked out, the pig, rhino, and moose didnt. So taking that horse, Grog and Nog started a breeding program, till Abdul came along, and started getting really fancy, The horses that carried us good got to live and breed, the ones that didnt made great steaks,
The 20 and 30 percent rules actually have a basis in fact on quite a few studies by the US Army, The 20% rule was for a forever load, basically it was found a horse could carry a 20% load all day every day for years without any ill effect. 30% for short campaigns, but the horse would wear down after a couple weeks, The way most of us ride only a few hours at a time once or twice a week, the horse will most likely be fine with alot more.
The horse in the OP may have issues that would limit that though.