Money. That's key.
I'd never really encountered big families, growing up CofE in southern England. I'm late 50s.
Then, in sixth form, girls joined my GS from the local convent, most from rather larger families, with various tales to tell. 4-6.
Then I found myself in Australia, Brisbane, in my mid 20s surrounded by established Catholic families of 7-11! Given that something like 20% of Australians can trace family back to rural, Catholic Ireland, this should have come as no surprise; but, like many immigrants to strange, new lands, 'tradition' was rigidly adhered to as a link to distant roots. So while the young Irish in Ireland were partying, getting contraception, thumbing their noses at the Catholic establishment; (and moving on); Australian catholics were clinging to the certainties of the orthodoxy prevalent in 1950, when they emigrated. Huge families, Catholic school, Mass every Sunday, plus.
I met many. And so many spoke of casual, uninterested parented, too-many-kids-to-care-about childhoods. But they, at school, were surrounded by kids in the same circumstances. It was the norm. Mid 20s adults who visited home, 2 miles away, once a month. Resentful of having spent their own teens room sharing, clothes sharing, battling for attention, babysitting and bringing up younger siblings; while watching the youngest, as tge eldest walked away (uni)- indulged, getting away with hell.
Conversely, among my work colleagues were 2 women from families of 5 and 6 who'd had a great time. Why? Money. Big houses. Own rooms. Home help. Private (non-Catholic 😉 ) schools; Overseas holidays.
Money.