I would know about it, since I attended high school in Kyoto (which is one of the most conservative prefectures in Japan). It seems a higher C-cup is the ideal size in many people's minds regardless of the sex.
The funny thing is, ratio-wise Japanese women do have a pretty big breast size. If the average height of girls were similar to those in Europe/North America, proportionally the average size would be at least 86.67 cm (34.12 inches)!
That's why I said they underestimate themselves. I dunno how many but I do find chest enhancing supplements from Japan very common. I find it telling me that women over there wanted things bigger.