For me it’s about the quality of life I can give me and my children.
This is part money, and part time.
I want to live in X way. First of all that means we both work. A bigger house is probably unaffordable.
I want to be able to afford to save, retire, go on holidays, buy what I want, fund anything my kids need (education, tutoring, cars, flat deposits etc).
I want to have time to actually meet their needs (physical, emotional, mental), and to meet my own -
to be able to relax, have hobbies etc.
I am also very conscious I will probably be supporting them (and even living with them) well into adulthood.
All of this means having lots of children becomes more unattractive, to the rational brain.