The Eurozone is a massive economic engine. Often depending on rounding, and who did the study will show a larger GDP for EU vs US.

There's little question the Eurozone is a massive economy, however it hasn't grown since 2007:

http://www.washingtonpost.com/blogs/wonkblog/wp/2014/08/20/w...

In that time the US has added $3 trillion to its GDP, going from $14.5 trillion in 2007, to a present $17.65 trillion or so. It's likely the US has reclaimed its title as the largest economic zone. Given the US is set to expand at close to 3% for 2015, and is growing much faster than Europe, that gap is likely to expand in the next few years.