Both of my parents were the youngest children of big families. I think they came to rely on getting a certain amount of family emotional support, and had to overcome a lot of fear of self-reliance when the parents and older members of their respective families died or became less supportive. By contrast, I've never expected any support or assistance other than parental, which was necessarily limited because my father was to some degree preoccupied with his own health (understandably so). It gives one a different outlook on life to know from the get go that you have to do it all yourself or it's not gonna get done. I also think that a lot of youngest children, including my parents at times, act like big immature babies and like you are supposed to love them even when they are being nasty to you because, well, the baby is always loved.
I've also met a few middle children who I had trouble relating to because they seemed to feel constantly shoved aside or ignored or somehow lacking in the parental attention that the oldest and youngest kids just naturally got. This is a situation I can't relate to - if anything I got TOO much attention sometimes. My solution for lack of parental attention would simply be to ignore my parents back, and that doesn't sound too horrible to me (nice break actually, as much as I love my parents), but I am not a middle child and I realize after having met a few that I don't "get it".
I also tend to be weirded out by people from big families because they really seem to enjoy having a group of people either physically around or emotionally involved with them. I tend to prefer having one-on-one conversations, friendships, and just plain time spent. It's hard for me to deal with group emotional dynamics unless I know everybody really well, and even then it can be weird - I basically have to know I am valued and loved and it can be hard to feel that when there's a five-person interconnected web going on. One thing I do run into a lot is the younger child of gender A who has a bunch of older or mostly older sibs of gender B. Sometimes I relate to these younger children better, because their being the only girl or only boy in the family makes them sort of like an only child. However, sometimes they strike me as just too different to relate to, especially when it is a male who likes to have a harem of women hanging around all the time because they remind him of his sisters.