That's an interesting observation Doe, and one I wondered about when I lived in the UK. I agree that 'as a whole' people in the UK are more conservative over weights, but at the same time we have all those wonderful short stocky native ponies who are designed to carry full size adults, or in the case of the Highland Garron then a huge great dead stag down out of the hills.
Since I've lived out here I have become comfortable with the view of weight carrying out here. I've seen bigger people ride and compete and the horses manage well enough.
The small strong well built, short backed well sprung Quarter Horse is every bit as good a weight carrier as some of native ponies. I'm currently well over 200 pounds again
but 15.1 hh of Mr G, grade Pintaloosa, who knows what's in the mix, carries me without missing a beat, he is just immensely strong.
Again it also depends what a person is doing, when I'm big I mainly trail ride, walking and jogging/trotting on mainly level ground, that is a lit less stressful on a horse than working in a school or jumping.