The debate about when China will oust the United States as the world’s top economy has been all over the place, with recent estimates ranging from one to five years, as opposed to projections of one or two decades not that long ago. Now the International Monetary Fund says China has already zoomed ahead. The IMF, which in the spring calculated new exchange rates for comparing output of different economies, estimates the size of the U.S. economy in 2014 is $17.4 trillion, while China’s comes in at $17.6 trillion. The speed of the transformation is breathtaking: As recently as 2005, China’s economy was less than half the size of America’s. Moreover, the IMF projects that China’s economy will be 20 percent bigger than that of the U.S. by 2019.