How did this happen?

First, the country is urbanizing at a historic rate, much faster than the U.S. did in the 20th Century. More than 20 million Chinese relocate to cities each year, which is more people than live in downtown New York City, Los Angeles and Chicago combined. This massive change has taken place in less than 50 years. In 1978, less than a fifth of China’s population lived in cities. By 2020, that proportion will be 60 percent.

China's cities have been transformed to make room for this influx of people. By some estimates, half of China’s infrastructure has been built since 2000, with new rail networks, interstates, dams, airports and high-rise apartment buildings springing up across the country. For example, the gif below shows how Shanghai’s Eastern Pudong District changed between 1987 and 2013. You can see why Spike Jonze chose Pudong as the setting for a city of the future in the recent movie "Her."

It’s a statistic so mind-blowing that it stunned Bill Gates and inspired haiku. 

Yes, China’s economy has grown at an extraordinary rate, and it has more than four times as many people as the United States. But the 1900s were America’s great period of expansion, the century in which the U.S. built almost all of its roads and bridges, the Interstate system, the Hoover Dam, and many of the world’s tallest skyscrapers. And China and the U.S. are roughly the same size in terms of geographic area, ranking third and fourth in the world, respectively.