The reason why developed countries have small populations

The more developed a society is, the higher individual requirements are and the more they pursue self-realization. Therefore, not all people are willing to have children, and people focus more on doing something. This is why, in developed countries, individual performance is extremely high, which makes society more and more developed, but the population does not necessarily grow.

Developed immigrant countries in Europe and the United States, such as the United States and Canada, have smaller populations because they mainly rely on skilled immigrants and foreign investment for development. Old developed countries like the UK have a small population base due to their history of aggressive immigration (discovering the New World, occupying Africa, etc.).

Developed countries like Japan and South Korea that were driven to develop by being invaded have extremely dense populations. This is because these countries themselves were born out of agricultural (fishery) countries, and aggressive immigration has not been successful. (Japan invaded China).

In underdeveloped agricultural countries like China, since the dominant support for agriculture is labor, they promote fertility, have a large population base, and are growing rapidly.

For an underdeveloped agricultural country like India, the factors due to its large population are very complicated. It stands to reason that it was also ruled by the British but was underdeveloped. My personal opinion may be due to their race and unique religious beliefs.