The Economist is a pretty well regarded business magazine worldwide, maybe the best.
Your market == number of consumers X the money they have to spend
China has 4 times the amount of consumers than we do. Thing is right now they are poor, if their standard of living increases 20%, that's a huge amount of money they can spend.
You'll notice that the Economist isn't saying China will have a economy that's 4 times as big as the US, they aren't even saying China will have twice as big of an economy.