Why do developed countries have a small population and developing countries have a large population?

The more developed the society is, the higher the individual's demands are, and the more they pursue self-realization. Therefore, people are not all willing to have children, and people concentrate more on doing something. This is also why the individual performance in developed countries is extremely high, which makes the society more and more developed, but the population does not necessarily increase. Developed immigrant countries in Europe and America, such as the United States and Canada, have small populations because they mainly rely on skilled immigrants and foreign investment for development. Old developed countries like Britain have a small population base because of their aggressive immigration in history (discovering the new continent, occupying Africa, etc.). However, developed countries such as Japan and South Korea, which were driven by aggression, are extremely densely populated, because these countries themselves were born out of agricultural (fishing) countries, and aggressive immigration failed (Japan invaded China). In an underdeveloped agricultural country like China, because the leading support of agriculture is the labor force, it promotes fertility, has a large population base and is growing rapidly. In an underdeveloped agricultural country like India, the factors of huge population are very complicated, and it is reasonable to say that it was also ruled by Britain but not developed. My personal opinion may be due to their race and distinctive religious beliefs.

Reference: Baidu Encyclopedia.