Well, I guess someone should start breeding 2500 lb drafts for trail riding. Because that is what it would take for me, a western saddle, and my trail riding gear.
I'm riding a 14.3 Mustang now, who is built like he has a little draft in him, and he is not only sound at age 20 (after me having ridden him for the past 8 years and being a rope horse before that) but he never has back soreness when I brush or put pressure on his back. I am very particular about saddle fit though and went through about 10 saddles to find one that fit like a glove.
While I would love to weigh 100 lbs less, that's not the case at the moment. And my horses seem to carry me just fine. I think it's a trade-off. I give them the best care possible, lots of love and treats, vet care, hoof care, the best saddle fit I can manage, and they carry my fat butt around for a few hours a few days a week. I actually think they probably work less than horses did 100 years ago. I also have 2-3 horses to alternate with, so that helps too.