The China-tops-Japan story is everywhere today. It's no surprise: It fits the familiar rise-of-China narrative (and the less sexy but also valid decline-of-Japan narrative).
The narratives are all about growth. As Japan's economy (like that of the U.S.) has wheezed through the financial crisis, China has continued to grow at about 10 percent per year:
Of course, total GDP is a function not only of economic output but also of sheer size -- the more people a country has, the bigger its GDP will be. So China's huge population is a a key reason its GDP is so big.
And while total GDP is a key measure of a country's footprint in the global economy, it doesn't say much about its people's standard of living. For that, the key is GDP per capita, a measure that strips away differences in population.
On that score, China remains far, far below both Japan and the U.S.: