what i mean is the chinese fire-bellys are often fatter than the japanese ones and this is a good way to tell them apart according to a book i read few weeks ago
humm, but that's not, actually the case. your book seems at odds with, virtually, all the reference material. (available to me) and the experience of those i know who keep one or the other. the Japanese variant grows both, slightly, bigger and is noticeably fatter.
This makes sense, if you think about it. as the JFBN spends more time out of the water. it would need more body mass to conserve the heat it has!! ONLY GUESSING HERE