It's a bit dubious to say "Europe is the largest economy in the world", because it's not really a single economy, more a collection of economies of varying sizes, with different currencies, laws and banking systems.
You also have to be careful of what you mean by 'Europe', because there's the European Union, the European Economic Area, Europe the continent, Europe as used in the UK (which often means 'all those other foreign countries but not us' :) etc.
The GDP of the European Union is about 16 trillion dollars; the United States is about $14 trillion: http://en.wikipedia.org/wiki/G-20_major_economies. What's a couple of trillion bucks between friends? ;-)
You also have to be careful of what you mean by 'Europe', because there's the European Union, the European Economic Area, Europe the continent, Europe as used in the UK (which often means 'all those other foreign countries but not us' :) etc.