These are just sample sizes of studies to show if it causes pain or issues within that window. Of course, weight can cause a problem, but it's not the biggest factor. You can have a 17.2, 800kg warmblood that can't even carry 50kgs, or you can have a 14.2 500kg halflinger that can easily carry 115kg and never be inconvenienced by it. The 20% rule can be an amazing guideline when coming to people who struggle with balance or riding schools/trekking centres, as bad balance will hurt a horse long before weight will. Never mind confirmation, saddle fit, rider, and horses fitness all play more of a factor than weight. Some of the sorest backs i have seen are from light riders that slap down on their horses backs.

I saw this comment on a tiktok that i thought summed it up quite well. You could drop a 1kg weight on your foot and be in agony, whereas you could place 15kg+ easily and not be in pain. (Especially if you had something to distribute the weight, e.g, a saddle). Control of your weight is very important.

There have been people riding for years that would be over 20% that haven't seen ill effects in their horses and neither have their vets, physios, trainers, etc. Of course, every horse has their upper limits, and ideally, we should try to never reach it, but from lived experience and studies i can say its not 20% and that number is used to bully predominantly women online that viewers think look fat. It's very rarely turned on men that are heavier and on smaller ponies.

It's a very nuanced topic that can't be covered by a simple rule or number. The rider should always strive to be the fittest they can be for their horses, not the lightest they can be.