Oh sorry if you got the wrong impression! I don't think that is what the point was at all, just that you should know thy horse, and the horses capability.
I have an arab, and my trainer is probably about 250lbs or so, and he carries her with ease. Canter and all. :) He can even manage some crow hops. ;)
It really depends on the horse and the rider I think.
Sorry if you felt bad, it wasn't the intention I am sure!!!
In fact, arabians are a breed that is known for strength and stamina! I have also heard that shorter back breeds support weight better.
"My treasures do not clink together or glitter; They gleam in the sun and neigh in the night."