As you must be aware, population growth comes from high family size which in turn comes from the need to ensure one's support at old age.

In extremely poor societies, where there are no pensions or social security for the old, the best way to make sure you have food and shelter when you're too old to work is to have many children.

And maybe in a society where everything is provided for you, the easiest way to find fulfillment is through having lots of kids to raise?

If you actually want to do a good job raising those kids, it certainly isn't the easiest way to find fulfillment...

But of course, in societies where you (can choose to have) everything (basic) provided for you, there are indeed some who have many kids but don't bother to actually raise them. This is how underclass is created.