China's population was only 180+ million when first they thought about population in the first period of 18th century. After 1851 they found that the population increased to 400+ million. But because of religious war for long time the population shrink to 370 million. Later it started to increase. After the war with japan a census showed that the population was 540+ million. Then the population boom upwards and in 1982 it touches 1 billion.
This makes China to get more population.
BEHIND THE POPULATION OF CHINA :
Population growth depends on birth and death rates, till decade ago the birth rate of Chinese was quite large, though now it is minimized by the government of China, the death rate of China have been less for centuries. Geographically China was completely secure from the south side in history because of Himalayas, China is good for farming so many people concentrated on the fortile land. The land never lost a huge citizens in 1700-1900 like Rassia or Germany. So, till 2000 the the population of China is increased exponentially & it become the largest populous country in the world.
The Yangtze river is the most fertile place in the world, this caused a massive rice plantations to pop up along the river and caused the population of China to explode early on with people settling over the banks of the river for hundreds of years.