That's a silly metric, though, because it just depends on where you divide things up politically. If China split into two independent countries, suddenly it wouldn't lead the world anymore, but the environment would not be any better off. Similarly, if Europe merged into a United States of Europe, its emissions would be much higher-ranked (maybe the largest), but it would not actually emit more than currently. The environment does not care at what scale you draw your political borders.
Per-capita emissions are one way of accounting for the obvious fact that if Denmark (a nation of 5 million people) generated as much CO2 per year as Germany does (a nation of 80 million people), Denmark would in this hypothetical be more blameworthy, and it would be fair to ask Denmark to reduce its emissions more than Germany, as it would already be emitting far out of proportion to its size. Put differently, if every country reduced their per-capita emissions to China's level, we'd actually be in great shape. That suggests that China is not the main problem.
Per-capita emissions are one way of accounting for the obvious fact that if Denmark (a nation of 5 million people) generated as much CO2 per year as Germany does (a nation of 80 million people), Denmark would in this hypothetical be more blameworthy, and it would be fair to ask Denmark to reduce its emissions more than Germany, as it would already be emitting far out of proportion to its size. Put differently, if every country reduced their per-capita emissions to China's level, we'd actually be in great shape. That suggests that China is not the main problem.