Uplander, you are right about forgetting the mpg -- unless you get a pop-up, the mileage is bad. But I respectfully disagree that a cramped trailer is always a mistake -- we got a very small trailer for our first one and discovered that it enabled us to boondock in remote areas with lots of trees and narrow roads. We got our second trailer, which is the same size as the first, for the same reason. We really enjoy it -- we camp between 60 and 80 nights a year.
Having said all of that, if you don't want to boondock and you don't need to fit into tight forest service campgrounds, the advantages of a tiny trailer probably do not outweigh the disadvantages, especially if you camp in a rainy climate and have to spend many days in very close quarters.