This one is from an ecchi anime series called "
MM!". A science geek girl with an undeveloped body for her age decides to take an aging potion in order to appeal to the main male character. Even though the AP is shown through her silhouette, the process is quite sexy. Not to mention that her voice contributes a lot as well. Sadly though, her AR sequence back to normal was not shown.
(17:38-18:12)
http://www.animeultima.tv/mm-episode-7/
P.S.
Since it's an ecchi anime, I was hoping that its BD would have the transformation uncensored. However, there was no change at all.