The reason why some places such as Africa and China have more population then others is because of poverty. Simple economics lesson says that if you have more kids, you have more workers who are going to funnel money back to the family.
Simple economics lesson also says that more kids = more financial burden, but not everybody's a genius.
Also, fatality rates among the poor are MUCH higher, if you're a kid in Somalia, you can reasonably expect to die. The only way to guarantee your genes carry on is if you have several carriers, that's a simple evolution lesson too.
It's tragic, but it's true.