Why are there so many people in China?

Why is China's population so large? I've been trying to figure out the ultimate cause and I think I have a possibility that is quite simple. First let's take a look at the proximate causes (the best source I could find about China's population is at Columbia University):
--From a historical perspective, China's population has always been large--it was already around 60 million around the birth of Christ (perhaps around 1/4 of the world, around what it is today)
--China's long civilization allowed for improvements in agricultural techniques and the clearing of large areas for agriculture.
--New crops from the New World increased food output in otherwise marginal agricultural areas

The ultimate cause, however, is...... RAIN!!! There is a reason why the diversity of species is generally greatest in areas of high rainfall and warm temperatures. Humans are also bound by nature and our density is dependent upon available resources dictated by climate.

I'm posting a population and rainfall map of China below. As you can see, they are very similar (source 1; source 2):




Of course, if you think about the history of human civilization, this all makes sense. From Cairo to Rome to New York to Beijing (Peking), all major cities are near a water source. I don't think I can name any large city in the world that is not near a river, lake, or ocean (except Mexico City with its large aquifers?).

Incidentally, India, with the second biggest population, also receives a huge amount of rainfall. The Himalayas block moisture from reaching Nepal and Tibet, much as the Cascades block moisture from reaching eastern Washington here in the U.S. I've never been to India or Nepal, but I can tell you that the difference in the landscape is amazing between areas west of the Cascades such as Seattle and areas east of it. The former is green and lush, the latter looks like a dessert (it almost is) except in irrigated areas. The population density follows the same pattern as rainfall.