It is inevitable that as any country becomes "developed", the size of their market will be highly correlated to their population. My country (the US) likes to think its something intrinsic about our people or society which has made us #1, but by far the most important thing going for us is a large developed population under the control of a strong single government.
It is no small feat to contain a 300+ million educated and developed population under a single unified society. Russia failed in the 80's, Europe is really struggling keeping the EU unified today, and the US went through a civil war and its hard to see a perfectly unified society in our current political climate. I believe China will have a harder time keeping control than any of my earlier examples, but even multiple fractured Chinese states would have significant economic power.
Projecting out world politics 50+ years into the future is impossible, but ultimately the largest and most unified markets will win economically. The winner could be a unified and developed China, a merged Euro-American market, or any number of other options. This is why the protectionist trend in my country is so troubling, but luckily these factions are rarely successful politically.