User blog comment:B214/Volume 23 discussion blog/@comment-1359322-20170220174230/@comment-31137559-20170301041621

@Grandpiece You should go back and read that carefully. It said Grayfia's body surpassed Rias'. It said nothing about Grayfia having bigger tits than Rias. It is possible to have a better body than another women while having smaller breasts. Not saying Grayfia's aren't big. But Grayfia being more physically matured than Rias is most likely the cause for that considering that Rias is 17 and Grayfia is around 500(estimate based on Lord Gremory's bishop).

For example, Issei is always saying that Rias' breasts are the best. For all means and purposes, Akeno's breasts should be the best considering hers are larger than Rias'. Yet it Rias' breasts that Issei loves more. In his mind, Rias' breasts surpass Akeno's. Not in size, but a lot of other things.