Total national physical resources are a constant. As population increases, the potential wealth per person decreases. This is why China will have a hard time elevating the standard of living of its entire population to the same level as America. Or India. Or any other country for that matter could have difficulty. They already have too many people.

It is a problem that can be fixed in a single generation (well, maybe 2 or 3). Just start trading birthing credits on the open market. You can set the total number of credits available every year to an arbitrary fixed number. Anything you want in theory. It depends what you want the average wealth potential of the people to be.