I’m not sure the two are related.
What happens when you’re poor you can’t afford to do anything so the only fun thing to do is have sex. And you’re not going to have protected sex either.
Then that huge family of 12 grows up and they have fewer children because they have a bit more money and an easier life and want to enjoy life. So they choose not to have a bunch of kids.
Then those kids grow up and decide they only want one kid at most so they can enjoy their life with all their money.
It’s the cycle of a nation.
When you can’t afford to have kids, due to excessive taxation, the rational thing is not to have kids. That is most people have done.