Im a westerner, and dangit I still have to meet the first woman here that is both 6ft+ and has cup-E
I think they do it to attract men to watch/read their stuff, as I cannot think of any story-related-necessity for them to look that way. I doubt that it's a way for japanese to express anxiety over their own body. It's a bit like how most movies show only beautifull people? It's a fantasy world, as the artist/director you can "idealise" or shape your characters the way you want them to look...