[a_c_s]

As developing economies 'develop' their per-capita GDP approaches that of developed economies. Therefore, as this happens, the size of an economy becomes more proportional to the population size.

Therefore over the very long term that absolutely should happen.

[adventured]

Not quite. Your position rests on the premise that China can bring its population up to developed status (something on par with the US, Britain, France, Germany, Japan etc), ditto for India. That is very unlikely to be feasible, given the truly massive manufacturing, consumption, exports, energy, resources, etc. that it would require to boost China's GDP to ~2x the size of the entire rest of the world just to bring their median income up to that of the US.