Why did japan and china got such a high population.

China has the largest population in the world, with a shocking number of 1.3 billion people and growing by the minute. ... Overpopulation in a country is the result of the number of people in an area being much higher than the country's available resources.

And for Japan, the Japanese population grew steadily throughout the 20th century, from around 44 million in 1900 to 128 million in 2000. The gains were primarily due to increased life expectancy, but also buoyed by families that typically had at least two children. ... By median age, Japan is the oldest large country in the world.

But that my opinion though.