Pathetically useless study, tells you nothing at all based on that sample - I used to work in market research, this would be laughed at for being so unrepresentative.
If you use the 10% weight ratio, then I am too heavy for Buddy, if you use the 15% ratio I am at the top end of what he can carry. He is a sturdy 16hh shire cross, with an 18 inch saddle, which is regularly checked, and I ride light.
Of course, if I let him gain more weight, then he would technically be able to carry me easier but in fact would find it harder. He has lost around 40kg this year as I have changed him onto full forage/fibre and managed to get more work into him.
I don't think the study was designed to be small.
I agree with your first paragraph there, as did Dr Dyson, but the conclusions publicised at this event we're that the rider weight made the horses lame, the fact the bigger riders were riding in saddles not fitted to them and on horses not physically acclimatised to carrying that weight appears to have been disregarded as they came up with a % weight based on this study alone. Now if each rider had ridden the horse for 3 months and had their own saddle fitted and the horses didn't cope then yes you could perhaps conclude it was weight alone causing the problems.
I bet if you went to any yard and checked saddles there would be plenty that don't fit. when I got Buddy, I had to have my saddle adjusted and wait a month for saddler, as I wasn't happy with it when I first rode him so I stopped. We began working at 15 minutes as I hadn't ridden in 8 years and now we ride every day for 30 minutes weather permitting and more at weekends and then 5-6 mile fun rides when we can find them. He carries me fine, has never been lame, and when we found he had a small rub under his saddle, the osteo found it was due to him being unlevel on one side and adjusted his pelvis and hinds, and hasn't rubbed since. It was interesting to see the correlation and the saddler had commented on it, we changed the numnah as well and keep an eye on it. I actually think the 18 inch saddle is bit big for me but I manage with it. He is full of energy and we keep an eye on his back all the time. But if you go simply on the weight ratios they quote then at 10% I am too heavy for him.