Its got nothing to do with the size of the country, look at Austrailia their population is even less than ours and yet they've kicked arse in several sports for many years now.

Its to do with the infrostructure and the attitude towards the sport. I'm sure the U.S will do well in time as you tend to be a very competitive people (sterotyping i know but it tends to be true) but I can't see it happening anytime soon.