Has lot to do with the particular campground, some the roads can't navigated with anything bigger, remember your truck and trailer bent in the middle Class A doesn't, well might after an oops!
Others like one of the state parks near me has very little room behind each site at most 1.5' and sometimes a few inches. Beach area ground is expensive!
They also may not keep the roads or sites trimmed back for larger rigs. So the site can be plenty big but can you get to it and into it.
One if you fudge on your size and you damage something it's on you. Most of us realize that and will take responsibility but there is always someone that comes on complaining their rig got damaged because of the trees and bushes and sometimes we find they told them they had a 5lb bag and really were dragging a 10 lb bag through the campground! :B