I so far was only able to find historical populations back a few decades. Both China and India's population have about doubled since the 1960's (despite China's 1 child policy enacted in the late 70's). The United States has somewhat lower growth rate; our population has doubled since the 1950's.
So, maybe the explanation is just that China and India have a larger population since they started with a larger population. That could make sense. Their cultures go back hundreds of years. Europeans in America goes back to the 1600's. The indigenous American population might have been smaller because it was behind, industrially. I'd have to have population records that go back farther, though, to confirm this guess.
A different explanation would be needed for the difference between China and India compared to Europe. Europe's surface area is slightly larger than China's, but is farther north. Hm. Even back in 1948, George Orwell wrote in 1984 of the "fecundity" of Eastasia.








