I’ve looked for that explanation, but I couldn’t find it. (Sorry).
I tend to like adjusting for scale, so per capita, share of GDP and more generally inflation-adjusted figures are preferable in my view.
Taking another poster’s link to the CIA factbook, here are the GDP figures (PPP adjusted):
1 World $ 65,610,000,000,000 2007 est.
2 European Union $ 14,430,000,000,000 2007 est.
3 United States $ 13,780,000,000,000 2007 est.
4 China $ 7,099,000,000,000 2007 est.
5 Japan $ 4,272,000,000,000 2007 est.
6 India $ 2,966,000,000,000 2007 est.
7 Germany $ 2,807,000,000,000 2007 est.
8 United Kingdom $ 2,130,000,000,000 2007 est.
9 Russia $ 2,097,000,000,000 2007 est.
10 France $ 2,075,000,000,000 2007 est.
Currently, the EU is less than 5% bigger than the US (after adjusting for purchasing power parity). So, I would say that the 2 markets are, “Roughly the same size”.
I would guess that the total energy consumption of the US exceeds that of the EU.
In general, I think that the US leads in plenty of fields (see my list of businesses for some examples), though often the social measures are a little low, when compared with other high income countries.