this as been pretty well beat to death here. basically there are those who have read too much, and ridden too little that will tell you, that a horse can/should carry no more than 20% of its own weight. There are also those who will tell you that as long as a horse is ridden properly with a balanced rider that weight is less of an issue.
the fact of it is, that just like people horses are all individuals, a lighter boned horse may be taxed at 20% where as a heavier boned horse may be able to carry 30%. the type of riding, will make a difference, a horse dog walking trails, vs. a hunter jumper will have different weight limits, a barrel horse will be different than a western pleasure horse.
Very little in the world of horses is black and white.