In poorer nations people have more children because, while life is hard, it doesn't get much harder if you have one more kid. You don't have to put your kid to 12 years of education; you don't have to worry about college fund. If you are a subsistence farmer life may actually get easier because a kid is a free labor.

That was also reflected in past Korean society where people frequently said the cost of one more kid is just "a spoon and a pair of chopsticks."

Very different from modern, industrialized Korea, where the lifetime cost of raising one kid equals multiple years of your salary. Unless you don't care about your kids and think they'll just sustain themselves by working in the rice paddy from age six to seventy.