What part of size matters do people not understand? It is pretty simple. The bigger you are, the more space you need.
If you are driving a 40'DP & you arrive in a no reservations CG & the only site large enough to accommodate you is occupied by a pop up or a class B, you are out of luck. The CG is also out of luck. It won't be getting any money from you because you have to go elsewhere.
Very simple. The smaller you are, the more options there are available to you.
We stayed in Gros Ventre CG, Grand Teton NP for 5 days last week. There were about 6 sites on the loop large enough long enough to accommodate us. The site across from us was barely long enough for a small class C. The office people were very good at assigning sites according to RV size.