This isn’t just girls, as in young girls, but really across the spectrum — old, young, big, small. It just seems like girls want to be tiny or worse, hide themselves. They hunch over slightly to hide their height. They pull their hands into their sleeves to hide their hands. They wear small flats and are slightly pigeon-toed, as if they want to hide their feet. They grow their hair long and have it hang in their face, or they pull their hair over the shoulder, trying to hide … something — their face? It seems they are doing their best to reduce the space they take up.
Does no one want to stand up and be proud of themselves? Why do they want to hide?
About Everything and Nothing 
I notice this, too. So does my mom (English professor) in her classes. It’s not all women, but it’s definitely noticeable. We still live in a patriarchy, and I think it will be a while until the system really changes in small ways (physical posture) and big ways (equal wages).
When I was a teen, I used to hunch over, not so much to make myself smaller, but to hide the fact that I had no boobies! I guess that amounts to the same thinking, I didn’t like something about myself, so I tried to hide!