Growth

I must say I should not be too excited with the historical record of China population, which you could easily find in the Wikipedia.

In short, the population of China fluctuated but remained around 50 to 100 million from the Tang (唐) dynasty to the beginning of the Qing (清) empire, but it shot up  in the subsequent 150 years and reached 400 million when Daoguang (道光) began his reign.

The obvious problem is why. In addition to the long period of stable society, most western scholars believe it was the result of introducing new crops from the Americas, including peanuts, potatoes and maize. Of course, as Chinese, we know these plants play relatively minor roles in our ordinary diet, and the extensive agricultural use of Champa rice (占城稻) was probably the main reason. This crop originates from Vietnam and was first sent to the Sung (宋) Emperor as a tribute gift from the Champa state of Vietnam. After centuries of adaptation and modification, Champa rice was gaining popularity amongst farmers because it is quick-maturing, drought resistant, and can allow two (occasionally three) harvests each year.

In other words, more land could be used for agricultural purpose, and the productivity doubles for any fixed area of farmland.

PS. The crops from the Americas, nevertheless, do play important roles. Potatoes and maize could grow in arid plateau and rugged mountains - places where rice could not survive. Peanuts, by the very nature of nitrogen fixing, greatly enriches the protein content of the ecosystem.