I wouldn't say that but it's true that in official media her butt tend to looks off.

>>712462939
Eh, as always it might be inconsistent. Like most Senran Kagura characters they have almost the same size, however, I think that canonically speaking Shiki's breasts are a little bigger but sometimes it looks like it's the other way around, it depends on the scene maybe.

The chest size of Japanese characters is something at least a little inconsistent in every IP.