I think you're right about the breast size correlation, for the most part. It's not like I think they're sitting there going "this character is a badass, we need to show it with 30lb tits" when they design characters, but I think they try to make badasses 'desirable' which obviously means massive breasts. No, I don't entirely get it but I'm pretty sure that's the logic.

Also, have you noticed the breast inflation factor over the years? Back In The Day, Naga from Slayers had craaazy huge breasts. So much so that it was almost embarrassing to watch. I noticed recently when browsing the shelves at Best Buy that, now, Naga is practically the size of your average shonen anime/manga character.

Oh, and almost all the best manga is written by women. Seriously.