In the world the average low income family has 5 children and the average wealthy family only has 1. 5. This matters directly because it means that inherited wealth goes to a shrinking population building the wealth gap. This also matter because NOT MOST but, Some poor people are poor for a reason (they are not too bright) this means that there are fewer smart people being born than dumb people. This is natural selection in the wrong direction and has likely contributed to the drop in IQ we have seem in modern times
It's because they won't be able to care and give a good quality of life if they have more than 1 child, Some of them will have to work at early age, Or maybe, Parents won't be able to maintence them and after that they will let kids in a garder o in the street
While, Sure, They will need to spend more money caring for more children in the short term if they have more than 1 child, Having many children is basically just an advantage. In countries with high infant mortality, Having more than one child increases the likelihood that at least one will survive to adulthood. On top of this, Having more children means, When they reach working age at about 14/15, Having more people who can contribute economically to the family and can do more work.