A lot of the transformations can be accomplished simply with the young lady working out in the gym and thus toning her torso and the services of a good make up artist.
Other performers are naturally pretty, even beautiful and simply do a bit of gym work. Ai Uehara was one who completely transformed her cute but not outstanding little bod into something quite remarkable. Quite recently I came across an early movie of Neo Akari (KAWD-794). She had the same beautiful face, but she was definitely carrying a few kilos more. I still found her desirable, but since then she's lost a bit of weight and in her later works works she looks absolutely stunning.
It's not only asian girls who have this ability of course, many western celebrities look quite plain without their war paint and some are so obsessed with the way they look, they won't even leave the house without an hour or two of preparation.