"The key point is the difference between income and wealth. GDP and related numbers measure income flows: namely, the quantity of goods and services produced in a given nation in a given year. Wealth is a measure of the total stock of resources in a nation and is much higher. Furthermore, the gap between wealth and income is usually higher for nations that have been wealthy and stable for a very long time, such as the U.S.
When it comes to national wealth, the U.S. has a big lead over China, possibly as much as three times greater. That is a very rough estimate by Michael Beckley of Tufts University, drawing on data from the World Bank and the United Nations."