system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do India and China have such high populations compared to other countries with large land size (like the U.S., Russia, Canada, and Australia)?

It boggles my mind why India and China each have over a BILLION more people than other big countries like the U.S., Brazil, and Australia. 

I understand that North America wasn't heavily populated prior to the European mass migration, but why is the Asian region so heavily populated compared to other continents?
gpt
It largely comes down to arable land. 

Canada and Australia just don't have anything like the right climate to support such massive populations and the agriculture required to sustain them. Here in Canada, a huge portion of the country is essentially a cold desert, and in Australia they have the opposite problem. 

The US is another matter, as there's a huge amount of high quality farmland. And as a result (well, there are other factors too, but this is a major one) they have the third largest population on earth.  So it's not as though the US is relatively empty or anything.  

China and India are both huge countries with ancient civilizations and intensive agriculture using just about all the available land in areas with large and fertile river deltas.  They've been among the most populous places on earth for a very long time now; this is by no means a recent thing.