Why are families smaller in developed countries

I heard families are bigger in developing countries.
Saddly, I think the reason is that they have to make a lot of children because they have some risks that children dies for illness or starvings.
Moreover, if they have a lot of children, the children can help house works and that helps the families!
In Japan it's becoming a problem that we have less younger ages.
Many married couple are too busy working and they don't make children.
Although I'd lo\ike to have a big family in my future.
I know it's hard to take care of a lot of kids, but I love playing around with them!