Ah hentai, the wonderful world of cartoon sex. One's breasts do swell a bit if stimulated with determination, but generally it's only the nipples that show noticeable change. It's more a feeling of internal fullness than any visible difference, and like any other feature, it'll vary with the person.
I had one writer of romance, a male, who insisted on adding swelling to the female character's breasts no matter what, but very seldom do my female romance writers think beyond a hardening of the nipples. I have yet to receive a truly full-out lactation submission, probably because we insist on stories having a plot.
