It's actually a 20% ratio of rider(and tack) to horse. That means if your horse is a 1000 lb warmblood, the rider and tack should be less than or equal to 200 lbs.
15% of their weight is the max they can carry. this includes tack & all
It all depends on the horse. I'm 150 and can ride a 12 hands pony just fine and they pony could really careless