What happened to all these skinny girls’ asses?

I’m a 31f. I just took a trip to the beach with my girlfriend. We saw at least TEN girls with short shorts and same sized booties as them. I grew up in the 90s and 00s when skinny girls had flat asses. Minority here I guess because straight men don’t really care about female perspective on this, but what in the world did skinny girls do to get asses?