system
Given the following post, respond as if you're commenting on Reddit:
human
Why did China and India become so heavily populated?

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gpt
A: They're very large countries to begin with in terms of geographical area. 
B: They both have multiple large river systems which can support large scale agriculture
C: They both have had centralized governments for millennia. Central governments, despite their bad reputation, do serve to allocate resources efficiently, which leads to greater food distribution and overall population health. This leads to more people, and thus, more children. 
D: This increased population millennia ago provided the base for today's population growth. Population growth is an exponential growth model, so a little population difference ages ago will multiply into huge differences in today's population. 

In summary, they had a head start.