Thought Id chime in here with a reality check regarding fibeglass and larger vs. smaller helmets. Lots of free form speculation in what we might call, "reconstructive" or applied archeology" that should be more carefully considered.
Generally speaking fiberglass will shrink about 1/32 per 10 inches if good quality resin is used. It will shrink up to -and a hair over- 1/16 per 10 or so for lower end resins. (so as much as you may think your glory helmet is perfect....its still shruken and made worse if its a second generation.....which it probably is
....but cheer up, its not that bad.
Now molding compounds can also shrink. However silicone molds, which present little to no shrinkage, were developed in the 1970s which almost certainly found use in studio costume houses since multiple production was often necessary and since Hollywood tends to be at the forefront of techniques for special effects.
So the issue at hand is thus....for a helmet that large to genuinely be smaller than another such that its truly noticible, it would have to have shrunk at least 1/4" or more...
....but there is caveat here.
These are not super thick structured pieces, nor is fiberglass resin a particularly stable material when curing...or worse, curing quickly under time constraints.
Moreover, the open nature of the vader mask leaves it susceptible to widening and narrowing (so too with an improperly backed mold) such that one cast may appear drastically smaller than the next if the mold was widened a bit OR if a cast was widened and then reenforced with more fiberglass. Variable after variable here guys
The Don Post Fett helmet was often thought of as a fake, and I argued ad naseum that its narrow appearence was the result of it being pushed inward at the sides....but consequently the back and front flared out a bit more too (big clue ;) Only after what must have been 20 casts did I manage to restore the original shape that is both convincing and presents a massively different helmet. I still recall people referring to the Don Post deluxe as 15% smaller than the original when it was still the same volume of material, just shaped differently.
So be careful when discussing huge variations regarding these helmets. My sense is that most owners of authentic helmets dont willy nilly allow folks to make molds, then those owners allow their helmets to be molded up and so forth. Recasting does exist, sure, however it would take A LOT of recasts to create a helmet that is so much smaller it would be immediately noticed.
Sanding does make something smaller. So do generations of recasting
(at least 4-5 to be blatently obvious).....but then again, so do variations in the inward vs. outward shape of the helmet ...A 'U' shape vs a 'V 'shape. ...the U shape appears bigger, more bull-like, more massive. ...but the V shape appears smaller.
Stuff like that.
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