1. Education.
The countries with higher level of education, where everybody can access to school, university, etc, have few children. Countries with the lower level of education have huge families. Why? Because the people with education are more conscious about what means to have a huge family, not only for the impact in the world, but also because they have other goals, like make a professional life. In the countries where the woman is still seen as a child machine, well, there you have 5 children per woman
2. Religion
The most atheists a population is, the fewer children they have. Meantime, where religion is on every corner, there you have again huge families, with less money and lower education.

So, in short:
Religion+low education+ poverty= a lot of children with no education and in poverty, also meaning: cheap workers for rich countries.
Atheism+ high education+money= few children with high education and money.