I would think it would depend on the condition, age, breed, etc. of horse as well as the skill and ability of the rider. I mean if you put an inexperienced bouncing butt rider who weighs 120lbs on a 1,100lb 30 year old arthritic, sway backed 16 hand TBH horse and they ride at a gallop for 50% of their 20 mile trail ride over rocks and mountains crossing paved roads etc. it would do more damage than a 200lb experienced rider with a good seat on a 7 year old well muscled 1,100lb. Fjord horse riding in an indoor groomed arena.
I think that it isn't realistic to make a blank statement as to a percentage of rider weight per horse weight do to all the variables.