Re: hitch size
Probably the shank diameter, the tow ratings are dependant on the shank size. I have 2 5/16 balls with ratings from 6000lb to 30,000lb, the 30,000 ones have 1 1/2" shanks, the 6000 ones have 1 inch shanks. Your 6000 is most likely a 1" shank, and the 10,000 is 1 1/4", at least that is how mine are.
