Generally, at least it used to be, 12" wheels and tires
had fewer choices for load ratings and styles. But for a real light
trailer, they may be efficient.
The larger the wheel and tire, generally you can haul a heavier load due to the heavier thicker 6 and 8 ply tires
available, design of the wheel itself, and air pressure. It's true Casita
has 14" wheels. 15" wheels fit right on to the new high lift Casitas as well. In fact others here on the forum have already made that switch. Yes, Casitas do have a 3-way reefer.
Lack of bearing grease and not checking tire pressure is the downfall of the system no matter what size you are running. I may be paranoid but I always carry extra bearings, grease, and a good spare tire/wheel. And of course the tools it takes to change both out. Some areas are so remote, I could repair (hopefully) the problem before help ever could arrive. It will be great when satellite telephones become common and inexpensive.