[vidgamer85]

Not sure if it was intentional or not, but I do believe they even joke about it in one of the 'skit' things at the end of one of the books (you know, the ones with that very odd "sketched on notebook paper in biology class" style).

Taken from the Wikipedia page on Seras:
A popular inside joke concerning Seras is the size of her chest. Not only is her endowment a source of ample discussion by fans, but it also seems that the author is in on the joke, too, having used a post-chapter comic to have the Valentine brothers comment on certain "growth" during the preceding chapter.