They can't get bigger than 8' 6" because that's the widest the law allows. Some states have or may still some restrictions on which roads 8' 6" vehicles are allowed. Various camping guides have the limits listed, though sometimes they are years behind.
In the newer ones, some of the smaller trailers are 8'—I believe the Sport (formerly Safari Sport) is and maybe the Bambis, but most of the mid and larger sized are 8' 5.5".
I don't know when the changes occurred, but you will surely have those answers shortly.