Another consideration: could it be that with more children, in an abusive household the abuse is typically spreaded out to more children, so that each individual child gets less abuse than in a smaller abusive household?
Anecdotal, but I think it's that one of the children gets the brunt of it and the others adjust. In my case, my elder brother had it the roughest, also since my parents were young and it was their first; I "compensated" for his more rebellious behaviour by not doing all the things he did "wrong". There's also something at my ex partner's case but I can't speak for her, but I can say that even though she got it worst, all her siblings ended up messed up in some way.
Every kid hands up having a separate childhood even in the same house, because the caregivers/abusers will change over time. Often one kid tends to take the brunt of the abuse, and the family roles end up reinforcing that.
My brother and I talk a lot about this, how we're only 5 years apart, but the first 5 years of my childhood and the last 5 of his are full of completely different types of abuse, and we weren't really treated the same during the years we were home together. The worst things that happened to me happened before he was old enough to form memories, and his were after I left the house.
Mine is 2 years younger but it resulted in post traumatic stress and high dissociation for me, and not nearly as bad effects for my sibling. We were treated different, had different roles, we are different in that I’m neurodivergent and he is not, and also he had a much more available social group of his age.
