I once shared a ‘car’ on the Space Mountain ride with a guy was was easily 350 (or more). Every time we went around curves in the dark I was freaked out about the G-forces on the wheels and tracks. I would hope they have considered this in their engineering, but it really bothered me.
Also, I've seen very obese people rent horses to ride. I think that is cruel, and there should definitely be a weight limit for this.