User blog comment:Godisme/Tartarus arc - Prologue/@comment-11267082-20131020135609/@comment-7418318-20131021115854

Considering Kyouka's comment it's more likely that the other 8's appearance was their true form, with only Silver appearing in an unnatural form.

Just because Doriarte's Human shape was still quite beastly and his demon shape was very big, doesn't mean the nine demon gates have to be the same.