Well, the war horses of old were around 15 hands high, and they carried knights in armor. The armor actually only weighed around 50-75 lbs, but added to a 150-180 lb knight and saddle, and you would be in the 250+ range.
Horses used for cavalry in the 1600s carried their own supplies with them, and I've seen calculations that these horses, around 14 hands, carried loads of 300-350 lbs on long days of riding.
One of the studies I saw said that experienced horses changed the way they moved when carrying a heavy load (>30%). They couldn't go as fast, but the adjustment meant their joints had no more strain than otherwise.
Here is some reading for anyone interested: Heavier Riders' Guide Horses How Much Weight Can Your Horse Safely Carry? http://www.distanceriding.org/php/ar...h/Function.pdf
My 835 lb gelding, who weighed less as a ranch horse, carried a >200 lb guy with 40 lb saddle for 8-12 hour days in the mountains, daily. He would lose weight over the summer, since there was no money for grain & he had to forage while being ridden, but he doesn't seem worse for wear.
Cavalry mounts during the Civil War would regularly be ridden distances of 40-50 miles/day for a week running. They would wear out if pushed any further, but that is use that almost no recreational rider puts their horse thru.
I'd bet there are a lot of cowboys who weigh 225 in their stockings, and they find horses to ride - although I'd bet their horses are conditioned better than a lot of recreational horses.
This photo is of my 5'0" daughter-in-law on Cowboy, but Cowboy is a 13 hand mustang who seems to have no trouble at all with my 175 lbs + 30 lb saddle...has a kind of weird gait, but based on the last 2 weeks, it looks like the little sucker will really go. Wouldn't put anyone over 200 lbs on him, but I'm told he has done it before: