This question is listed as one of the 'Popular Questions (FAQ)' on the sidebar. Please see the following two threads.

• "Why has China historically and continuously had such a high population with regards to other parts of the world?"

• "From a historical perspective, what is the reason for the relatively high populations of India and China, compared to places like Europe and South America, for example? Were the Indians and Chinese of the past comparatively healthier than people in other areas?"

The TLDR version is that the large populations of China and India are attributable to their high agricultural output.