This is because when countries Westernize the birth rate goes down.
I think what you are referring to as "westernize" is to adopt the long term leftist goals of "free love" (responsibility free sex), Abortion, and homosexuality. These things have decimated our population.
The left have always been Malthusians.
When we were a rural nation families needed to be larger.
This was because the the technology was not as great compared to today. Thus, not every family meme ER would make it.... Plus you needed more people to run the farm. You may lose some that leave the home too. That’s why you needed big families. We still see that in many rural societies today. In Western societies you don’t see that because of technology.
Although I believe generally speaking there is a correlation between morality and freedom. The lower the morality in a nation the less free society become. The higher the morality in a society the less crimes you see. The more free the people become.