They are not often skinny in the way that I would define skinny but if you mean just not overweight or obese, compared to other parts of the world the diet is filled with fiber like vegetables and probiotic foods like pickled vegetables, so they have both good stomachs and plenty of digestion aiding foods, although i think in today's japan highly processed instant food and junk food is rising in popularity due to lifestyle changes over the decades so I think Japanese girls might be getting a bit chunkier. Still not to the point of obesity
Here are some popular gravure idols that are known as being plump:
Chitose Yoshino, Ai Shinozaki, Hitomi Kitamura
Rei Ikeda, Airi Igawa and Yui Kudo are heavier amongst idols
Shiori Konno is fatter with every dvd release
On the skinny side I'd categorize girls like
Fuchigami Hikaru, Maity, Momoka Hayashida
Small limbs, flat belly, minimum body fat
Its cherry-picking from gravure idols yes but I think the common Japanese girl is probably just average, they are predisposed by genetics to carry weight around the middle which is a bit easier to hide when your job isn't taking pictures in a bikini