Everyone being "happy" and "getting along" may not always infer the types of life skills that one might want for adult children (and not meaning to find any fault in anyone's family makeup). It may well be the case that peaceful families (as defined by parental perception) are not the best preparation for life.
I think this is so true, at least in my experience. I'm an only, and I do feel like I missed out on learning a lot of social skills at an early age, compared to my daughters. But honestly, I think they gain most of these skills in moments when they're fighting--learning to resolve conflicts, learning to tolerate difficult emotions, learning to get over it when they're really mad about something, learning to put up with someone who doesn't act in ways you want them to (ie, a younger sibling who breaks things or spills things or refuses to pretend to be a pony when you want to play ponies) etc.
I don't necessarily think this has much to do with gender, but I do think that larger families offer children more opportunities to develop tolerance and resilience, even--and perhaps especially--in moments when they're not feeling "happy."