Size of the RVs I think has less to do with these out of date CGs than what people's expectations were years ago.
Most CGs and RV parks that were built "from scratch" in the last 20 years are pretty much built for the modern RVer. Many no longer have any tent sites, they have 50 amp hookups, level driveways, nicely manicured lawns, squeaky clean modern shower facilities, etc. This also goes for remodeled campgrounds where someone had the cash to rip out sections and start over. But these are things many RVers expect today, more cushiness, more convenience.
Unfortunately a great many CGs were built at a time when camping, even with a trailer, meant roughing it. The primary concern was having a spot to put the trailer in and an electric connection to run the lights. They didn't have microwaves, and A/C, and entertainment systems. Dump stations were done as a convenience more than a necessity. Full timing wasn't as popular as it is now.
We frequent a CG that was once a KOA. The current owners added an RV park type section on land they own. Its everything I cited above about new CGs. The "old" part of the CG, while it has upgraded electric, water, and newly leveled sites, very clearly was designed for the old expectations campers had years ago. Lots of small sites, odd angles to back into, odd location and tight area for dump station, etc.