How old are you ?
Because bodies tend to change as we age.
It looks like Ariana does have somewhat of a child's body, with no boobs but she's got a little butt.
I'm 18 and my body has changed so much from when I was 13.
You lose your baby weight and you will most likely have some sort of hour glass shape.
If you are really self conscious, you can speed up the process by exercising.
People say you need to so squats for a big butt, which will no doubt work but I have back problems and cant lift weights so instead I just go long distance jog. It really makes your thighs and butt more muscular. You can start by running 2 miles slow, then try to get it faster (over the course of like a week, don't try to run to miles fast starting out, you might pass out) and you will start to feel the burn in your butt.
Oh and yoga helps with getting a more feminine shape too!
Just don't do the kind of elliptical that you sit down on, like a bike, because it will just make your butt flatter.