Agreed with most of the above. I worked 2 years on a trail riding farm, and it is the horses. Some just simply cannot be expected to carry the weight of some people, especially since we tacked up at 5AM, and didn't get the saddles back off until around 6 that evening. Even with rotating horses, doing this six to five days a week is a lot of work.
However we had larger draft and stocky QH's on the line for the bigger riders, but we still had to be careful about who we booked so that the thinner built horses didn't have to overload.
Farm liabilities (riding farms) usually cover everyone; because lets face it, people fall off. You can try to avoid it all you want, but someone is going to hit the ground eventually. However good trail horses are hard to find, the ones needed where you can throw anyone up and they take care of you... so of course you don't want to ruin that horse by overburdening them and breaking them down WAY before they should. It's just good horse sense.
Horses and riders are matched up on these agendas:
How well we think they'd work together.
How well the horse can carry (comfortably, if possible) the rider.
And what the rider expects from their ride. (We had spunky horses, quiet deadbroke horses, ect. It was a giant puzzle to put together every day)
If you have trouble finding a lesson barn that will accept you; invest in your own horse that can comfortably carry you and take lessons on it. Really, expecting any horse to break its back just because someone wants to play around on it is selfish, and I wouldn't have it. No one over 180 is allowed on my mare; period.