Scrawny little Scootaloo is pretty much grown up, but her body hasn't reflected her age and she still resembles a scrawny little kid, much to her personal chagrin and embarrassment around her other, more matured friends. Rainbow Dash promises to kickstart her metabolism... and the results really start to show after a couple of weeks...

Inspired by/spiritual successor to a fic by D-Tech, which can be read here.
(While not required reading, if you like this kind of story I would recommend it.)