bakunyuu

A pair of very large breasts.

In a common hentai video, the viewer will notice a few things: larger than possible—even with plastic surgery—breasts on women (Bakunyū), whale-sized penises on males and transsexuals, tentacle projections used in erotic performance, incredible amounts of sperm often covering whole rooms, hermaphroditic characters with enlarged female and male parts (futanari), and/or Lolita/amateur characters (lolicon).[…]

A genre of Japanese pornography (typically hentai) featuring women with this type of breast size.