China has had 4+ times the population of the US for a very long time. The real reason they haven't been ahead of the US in production for many decades is that the productivity of the people was squandered by political forces within the country. It was corruption at the highest level, trading the productive potential for political stability.
We have corruption too. Bad politicians who do sweetheart deals with contributors, crappy patent or copyright laws, lawsuits over unreasonable things with unreasonable settlements and banking malfeasance are all examples of our elements of corruption and there are many more. I think it's relative corruption that will decide if one population is a dozen times as effective as another. That means it will decide who's economy runs things in the 21st century.