I think this study is on the right track. They're acknowledging that horses have different abilities, and they've found a method to determine what is too much for each horse.
I don't think that percentage would be right for every horse though, but that method of figuring it out definitely sounds promising. The quote under the photograph says it all -
"While the horse used in the study topped out at 209 pounds, certain types of horses can handle more (or less) loading, Matsuura noted. Ideally different breeds should be evaluated for maximum weight loading recommendations, he added."
Trust the Japanese to know how to get it 'right' xD