So the argument of "more to choose" from doesn't really account. The US simply has better athletes because they train harder, not because there's more of them. While the US has 647 athletes to China's 396, China is actually ahead on the current gold count, but trail in total medal count by only 5 medals. While China only sends 61% of the number of athletes to Athens compared to United States, they certainly have much more than 61% of the medals compared to the United States.
You look back 4 years ago in Sydney, Australia sent close to 500 athletes in Sydney compared to the US who sent about 620 athletes. The US won the total gold and total medal count with 97(39 gold) total medals. Australia only had 58 total medals with 18 gold.