User blog comment:B214/Question and Answers 2/@comment-24589341-20140330092032/@comment-16281343-20140330143350

The illustration in Volume 13 was an error because Ravel was drawn with breast size near Koneko's. But in truth, hers is slightly bigger than Asia's.

Look at the illustrations in her image gallery. All illustrations except the one in the sauna with Rias and Akeno has her breast drawn correctly.