Infant and child mortality was very high. Empirically the high birth rate did not always lead to overpopulation - overpopulation waxed and waned despite the persistently high birth rate.

The sci-fi dystopias about overpopulation never really come up with a mechanism by which the birth rate remains high despite low infant and child mortality and low economic need for human labor. It always seems to boil down to "people are controlled by irrational instincts".