I think its simply a matter of the smart campground owner perhaps encouraging a small rig to use a smaller site, leaving the larger one open - it increases my potential customer pool. That said, however, if there is a site that pop-up owner is in love with I'd accomodate the 'bird in hand' as the prospect of any additional demand for that site is purely speculative.
Most of our public parks require a tent or RV be on the site if it is rented. In other words, you can't rent a site for a week and only show up for a couple days. Of course that doesn't keep people from parking the rig on a site for a week and only using it for a couple days. I used to see that a lot at the EAA campground in Oshkosh. Folks would bring the RV in weeks in advance of the convention to get a prime site. After setting up they'd then go home, returning only for the actual convention dates.
Jay & Peggy Monroe with Dolly
Can't take it with you - don't plan on leaving any behind
2016 Newmar London Aire 4553, Spartan chassis
Wrangler Unlimited toad