I've only had one singleton litter; that F7A baby was very large when he was born, and as he had complete access to the milk bar, he was very big and plump. For some reason, his mother decided when he was 7 days old that she wanted to raise him in our bed
He was very spoiled I think with all the attention he got from us, but very confident and outgoing as well. I would even say he was quite cocky, too, and when I get updates from his owner, he is still a big boy, very confident and intelligent, and made friends with the resident dog very quickly, as well as being super friendly with visitors. He was handled by my grandchildren (as all my kittens are) , so was exposed to a variety of different people handling him before he left, as well as going to shows as a companion to my show kitty, trips to the pet store and visiting shut-in elderly people, playing with some of my adult cats (my older neutered boy is wonderful at teaching the babies "kitty manners"), plus lots of exposure to loud music by my fiance
I have honestly never had any of my kittens be "jumpy", whether a single baby or one of 5. I believe that early and proper socialization is one of the most important factors for any kitten, but I have heard of a few kittens who are a bit shy and reserved even with the same socialization as their siblings; although they may grow confident with a wise and patient owner...