Coming from the youngest child in the family (a girl among 4 boys), I've had more advantages. I'm sure having such strong relationships with my brothers has helped significantly in the long run, but I've gotten far in life because of it. Tutors were free and immediately available, I had immediate advice regarding anything from high school crushes to what classes to take to how to deal with depression productively. Sure, they were still stupid sometimes and frequently spent more days going to parties than attending college, but I used their mistakes to my advantage.
Next to all that, I had consistent support. I know it's pretty stereotypical for the big sibling to hate the little one, but if nightmares came up or I was afraid of what I thought I heard or saw, they took care of it. I've slept in their beds due to fear, they've taken knives to their front doors when I was scared that police were outside and I thought something dangerous was happening. And maybe it was just that my dad wasn't the best guy, but they made up for the fact that I had a single mom and very few friends - even though they were getting into careers, getting married, having kids, or whatever.
Other than that, it was purely an advantage to be younger because I got unnecessarily spoiled. Just for a birthday I received a laptop and new cell phone, supposedly because I was "a teenager with growing social circles".
As for family size and it's influences, I do believe I had some advantage in having so many brothers around, even if they didn't all live with me at certain points. I mean, obviously, if one was too busy to cope with the ridiculous things I had in my head, another was bound to be doing nothing. My system went four different directions, which could be vastly different from someone who only has one, or even two. My status rose as well, and part of that may have been that they didn't all get along (they cope, though). Even if they hated each other at certain points, it seemed like they all had a very clear goal: protecting me, or being there for me. For some reason, I had a big significance in their lives, and it didn't fade as any of us grew up (do brothers usually get their sister's name tattooed on them? Because that happened here. Twice).