Arable land is a problem. Most of China lives in one third of the country, and even in that one third there are a lot of mountains that keep population even further concentrated. Russia has never figured out how to really population Siberia and it’s FarEast, it’s sort of a hostile area, although the areas next to China could easily be as populated as the adjacent areas in China. India has a lot of arable land for its size, which is why it is populated the way it is, and see Bangladesh for an extreme take on how dense a country can get with enough arable land.