GB has a larger team because it a) made an enormous extra effort and b) automatically qualifies for every sport it enters. Sports such as waterpolo, volleyball, football, and hockey typically admit only the world's top 12 or so of countries. GB probably has an entry in all of them. Being team sports, that adds up.

The interesting thing to see for GB is how far they will fall back in ten years time. They were abysmal in 1996 (one gold), but grew since.