I can talk about India. India has the world’s largest Arable land. Both America and China are three times as big as india but still hold less or similar kind of Arable land. Excellent weather, almost all the places are inhabitable, very few extreme weather issues, long warm water coasts, predictable rain due to monsoons, well defined seasons, three to four agricultural seasons, etc. All these adds up easy food and shelter resulting in high population. India has been highly populated since almost from the advent of agriculture
Indias population probably declined significantly after the climate changes of the early 3rd millennium BC, as people have pointed out.
Growth got pretty significant during the second urbanization period, where you had these “Janapadas” or kingdom-republics forming across the gangetic plain around 1000-600 BC.
These eventually consolidated into 12 major Mahajanapadas or great kingdoms by 600-400 BC or so, and only here did you had a massive population expansion in the north that put India on the level of China.
Free labour. If these countries urbanize, they will go through the same population declines seen in western countries. Children offer free/cheap labour in agricultural areas. Therefore their production is outweighs their consumption. Once children stop being an asset and become a liability, that is they consume more than they produce, people have less of them.
Population is mostly a function of how many babies you can keep alive to breeding age.
The main things that limit this are disease and famine. Since most people farm their own food historically, more kids meant more production/food/money. It also meant that when one fell to disease, you had backup children.
In the west, people started to move away from farming significantly, around the 1900s was when technology and culture reached a point where having more children was not advantageous, as medicine kept kids alive and people were buying food not making food. However, culture and technology grew together in the west.
Around the same time in China, all this technology was brought to a culture that had not seen any of these changes in ideas. China had big families, with a population of 500mil even in 1900, so all of a sudden you had a whole generation of kids in large families, that didn’t die, and then they each had just as many kids.
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