The way you reduce family size is by increasing the standard of living, reducing the child mortality rate, and increasing access to education and birth control.

In poor countries children are your retirement savings, so at an individual level having more children is a rational decision. In many cases they aren't having more kids than they can afford--they can't afford not to have more kids. Even at a national level it's not clear that lowering birth rates without additional changes would help a country economically.