China’s rapid growth over the past 30 years has pulled hundreds of millions of Chinese out of poverty and turned China into the world’s factory floor. But China’s per capita gross domestic product is still just $4,300, according to the International Monetary Fund. It is largely because of the country’s population of 1.3 billion that China is moving to the top ranks of economic powers. On Monday, it formally surpassed Japan when Japan reported its 2010 GDP. Look at the arithmetic. China has 11 times Japan’s population. That is enough to propel it ahead of Japan in the GDP rankings, despite a per capita income of little more than one-tenth the level of Japan.