The International Monetary Fund (IMF) reports that, when calculated according to purchasing power, China now boasts the world's largest economy.
The calculation method is key here, because in raw numbers, the United States' GDP ($16.8 trillion in 2013) is still about $7 trillion ahead of China's ($9.2 trillion in 2013). But because of significant cost of living differences between the two countries, those figures don't provide a completely accurate picture. That's where purchasing power comparisons come in, and where China's growth is evident: By 2015, the U.S. will account for 16.28 percent of the world's purchasing power adjusted for GDP. China will be responsible for 16.48 percent.
China's growth is representative of a larger trend of "emerging economies" commandeering an ever-larger share of the world economy while "advanced economies" become less prominent.