Lower income people often trend towards the former, higher income people towards the latter. Of course there are exceptions - some lower income people dedicate great resources towards their kids, some wealthier families don't.
The opportunity cost as well. Having and raising a child means there are things you can't do because you have to raise/take care of the child.
> If someone lives in a place where housing is cheap and don't expect anything beyond public primary and secondary school then another kid only really costs some extra groceries and clothes.
At the end of the day, even for people with low COL raising a child to adulthood is still on the order of tens of thousands of dollars. Planning might also be a factor there, it's likely more present higher up the income ladder, meaning the wealthier segments see the kind of costs associated middle/long term and go "maybe not yet".
Raising a kid to the expectations of an upper or even upper-middle class family costs orders of magnitude more than raising a kid to the expectations of a poor family. But said upper middle class family probably doesn't make a proportionally larger amount. I'd consider someone "poor" if they make 20-25k a year and "wealthy" if they make 200-250k+. But the costs of raising a child to the expectations of the latter is probably more than 10 times greater. Probably 20-30 times greater. Thus why kids are more proportionally expensive for higher income class people.
The best predictor for someone’s economic security in life is the zip code they grow up in.
Of course there are people who don't have much money and still do their best to raise their kids as best they can. But the reality is that things like education and upbringing do have a significant impact, and often getting the best out of those things takes a significant mount of money.
Hence, why Westerners are waiting to have kids and dedicating more resources towards them. Furthermore things integral to raising children like education have actually increased in cost over the past several decades.