We learnt that he who shouted loudest, who was the 'problem' got attention.
Even in small families this can be true. I watched my friend struggling to sit both 2 yo twins on her lap the other day. The more demanding, Diva-like girl twin kept screaming and kicking the easy going, quieter boy twin off Mummy's lap. Boy twin got handed to Granny on the basis that he would soon stop crying and accept his fate, whereas Madame Diva would let all hell break loose if mummy dared to choose boy twin over her (and wasn't prepared to share mummy's lap either) so life would be unpleasant for everyone in the room for half an hour if she didn't get her own way.
I'm not blaming my friend. We all take the path of least resistance with our children from time to time, but it's important to recognise when the balance has tipped too far in favour of the squeaky wheel, and sometimes only outsiders are capable of seeing that. I have three, and at times I have had to leave two of them to get on with it, while all of my energies were concentrated on the other one, who has been more labour intensive.
If your life consists of spinning plates and firefighting just to get through each day, you sometimes don't see further than 'they were all fed, half of them got dressed, some teeth were cleaned, no-one needed to go to A&E, phew, that'll do.'
But it's the little inconsistencies and unfairnesses, the bullying that goes unnoticed, the pain or the worry that gets dismissed, these are the things that shape people in later life, and give them their lifelong insecurities and hang ups, even if people don't always consciously know why.
My friend's DD may grow up always expecting to be prioritised, like a hissing pressure cooker that might explode at any minute if you don't drop everything. Whereas her brother may sense that his feelings are not always acknowledged, so he will become very good at suppressing them and will deal quietly with the big stuff on his own. Neither thing is terribly healthy! Work will need to be done to ensure they both turn out well balanced. It's the same with all children in all families.
But this is in a family of two. Imagine that x 4. Eight children with different personalities all vying for the attentions of one or two adults. You may have 8 lots of unlimited love, but you don't have unlimited energy or time or patience. You don't have extra arms, a bigger lap, more hours in your day, more eyes in your head. You don't function perfectly well on less sleep than any other normal human being. You just don't.
In mammals who have large litters there is always a top dog and a runt, if you can forgive the dog analogy for a moment. Then imagine those 8 children don't all have the same father, and some have deep seated issues of rejection or inferiority because of feeling usurped or sidelined to make way for new babies with a man who is not their dad.
Imagine being told you are really talented at athletics and invited to train for county level, but it means going two nights a week to a town 15 miles away and lots of weekend commitment, and mum/dad won't take you because they are too exhausted from just trying to hold it all together at home. Or because they can't spare the time or the expense and it's not fair on the others. 
Of course this could happen in any family, but it's more likely to happen in large ones.
Imagine being the eldest sibling and being sick enough to need the doctor, but mum doesn't take you because she has no-one to watch the younger seven and she can't face dragging them all out, so you suffer for a day or two longer than is right, or fair.
Imagine being a hormonal teenager who needs privacy to study or just think, and having to share every waking moment of your day with a huge bunch of kids under ten who want to wrestle on your bed and play with the few precious belongings that are actually yours and yours alone.
I don't get on with all my brothers and sisters, but my DC are very different personalities and get on like a house of fire.
I think everyone who has chosen to have a very large family (and by large I mean 5 +) tells themselves that. You need to spin it to justify it. Of course some large families are far happier and more functional than others, I'm not doubting that. Healthy finances will take care of many of the practical issues but not necessarily the psychological or emotional ones.
'I chose to have 6/8/10/15 children in spite of knowing that it might impact negatively on them, financially or psychologically.'
said. no. parent. ever. 
I don't expect anyone with 5 or more children to admit that it's anything other than one big, happy, well oiled machine where everyone's needs are met 100% of the time, round their house.
But their children, upon reflection, may tell a slightly different story in a few years time, as has been borne out by this thread.