Many double digit families that are doing an “amazing job,” are doing so because the mother has foisted the care of the younger children on the older one. And before anyone argues with me about that, I witness that personally ALL the time.
And a very real issue is that the children who are charged with this parenting responsibility are sometimes still in need of strong parental supervision themselves. I see six year olds watching 18 month olds by themselves. I have personally run across the street (and with my osteoarthritic knees that is no small feat, pun intended), to snap up that baby, as he was about to run into the street (and I am really tired of feeling responsible for the safety of these children whose parents are just not watching them).
The older girls (somehow it is never the boys) who end up caring for their infant siblings, may be getting experience at child care, but they are also often getting burned out at a very young age, being forced to spend most of their free time babysitting. I am sure there are exceptions to every rule, but on the whole, my observation is that this is what happens in many larger families.
I am not saying that all of the above cannot happen in small families, but it is less likely, based on what I see every day.