With regard to the GMH, there is a distinct size difference between it, and other known helmets.
Those that I have any knowledge or authority in :
"The Mystery Helmet" (MH) :
In most respects, The GMH is "different" from a Mystery Helmet. Meaning, larger in some areas, smaller in others. The reason is quite clear. The Mystery Helmet is still a bastardized idealistic interpretation by Don Post Studio's, half way between the real Pre-Pro 2 helmet, and the DP Deluxe. What that company did to it is shameful from the perspective of a preservationist. It shouldn't even be debatable.
"Movie Sized Helmet v.1 / v.2" , Fettpride Helmet (FPH) :
The GMH is smaller than an FPH, an original MSH or MSH2 for one very simple good reason. They were fan sculpts based on an idealized vision. A mere interpretation. And partially due to misinformation given us in 2003 about the size of the real helmet given by a so called "authority" on the matter as well as some mis interpreted laser measurements.
"Master Replica Helmets (MR) :
The GMH is smaller than the MR genre because Master Replica's "upsized" the helmet after their digital scan. Some reports said up to 12%. This was in consideration and anticipation of the manufacturing process and
shrinkage". Unfortunately, whatever number they started with, it was too much, as the MR series has been measured out to about 7% larger than an original ILM helmet after manufacturing, production, and recasting.
Fact : Screen accurate Boba Fett Costume components are MUCH smaller than anyone gives credit. Maybe it's because we want to believe different, or because we ALL want to fit in the suit. The truth is, only a fraction of us could actually pull off the suit. Remember how much "beefier" Don Bies looked in the suit compared to the
ESB actors in the pic up shots of SE ? Has anyone met Don Bies? He's not a big guy by any stretch. Though, like me, he has broadened over the years lol And let us not forget, that Jeremy Bulloch, although actually standing 5 ft. 10, was only approximately 145 lbs during filming of
ESB.