Keep in mind that "a Bitcoin" doesn't exist in any real sense in the code. What does exist is the satoshi (.00000001 BTC). In other words, there's not really such a thing as 21 million Bitcoins. Rather, in the technical sense there are 21x10^6*10^8=2.1x10^15 = 2.1 quadrillion satoshis. In comparison, according to Google, the world GDP is about $56 trillion, which is to say about 5.6 quadrillion pennies. (Yes, I know the whole world GDP is not actually measured in dollars, but go with me here.) So really, it's not that ridiculous a level of volume.