It is easier to find a 13", 14" or 15" tire if you ever blow one out on a weekend. I agree with the smaller bearings having to work twice as hard going 80 or 90 miles an hour down the highway, lol, thats nuts, they build up heat alot faster as was mentioned. I have one boat trailer with 12" tires on it, I never did like how it trailed on the gravel road. Personally I would not go below 13" unless you are not going to go too far from home.
Kilroy