Without going into a whole huge story of everything, I will try and explain. The mystery helmet was based off one of the pre pro or production helmets, if I am not mistaken(forgive me, it has been forever since I recalled the history). So Sarges mystery is gonna be very close to the accurate size. The rubies helmet, a gawdawful helmet, is in no way an accurate helmet in size. I own quite a few helmets, one being a Don post, a SGT Fang, MLC and a Boba Maker.
The SGT fang is bigger than the rubies. Rubies is actually a shrunken down helmet. So, if you got a SGT Fang, you got one fine helmet. I don't have any exact measurements, I think TK409s site may have some more info on that.