RealTechNews kind of misses the point here. It's not population per se that dictates CO2 emissions, it's industrial output, automobile use, etc. At least, so the theory goes as I understand it.
What this really shows is that the more advanced the industrial society, the more technology it can bring to bear on the issue. China is much more heavily reliant on burning dirty coal, hence their #1 ranking. One should also remember that the Chinese economy is still only about a fifth (or maybe a bit less) the size of the American economy, and so their CO2 emissions are even more egregious. Where the US to be as backwards in energy use as the Chinese, our own CO2 emissions would be stratospheric (no pun intended).
That is, to the extent that one believes any of it's meaningful in any sense.