I don't buy that more people on a site is more work and therefore should pay more.
Do the math. 4 sets of feet vs. 8 sets of feet = more stress and disturbance on the ground. More trash to dump (even if it's in the barrel). More weight on a picnic table means it wears out faster. Aside from the "work" angle, it stress resources which cost more. More toilet flushing, more tooth brushing and more dishes to wash equate to more water usage whether or not they use the cg showers. If they don't, more tp used in the cg bathroom, etc. How often do you replace an item of clothing you wear only occasionally vs. one you wear daily?
In simple terms if you are having people over for dinner do they share plates, napkins and silverware? No, of course not. You use more. Extra dishes fill up your dishwasher faster so it may require you to run an extra load. Extra load means more water, more soap. It's subtle but it's real.