Obviously, insecurity and low self-esteem are nothing new, especially among girls. They're not even rare. The causes are usually something along the lines of domestic abuse, teasing in school, seeing those models on the runway or busty airbrushed-looking women in action films and feeling inferior- that sort of thing. But I've noticed the most "confident" girls who show how "proud" they are of their bodies by advertising them in various are nearly always the worse.
From my experience and observations (and seen and experienced this for quite awhile now), it's almost always overweight girls and "curvy" girls. (See pics below. I've already explained the difference between the two. I mean girls that have big boobs, wide hips, and some kind of smaller waist. Or at least they consider themselves curvy.) It's almost ironic. The latter will often claim that men prefer their body type because it's womanly, yet display themselves so much out of a miserable craving for attention.