This is going to sound dumb, but bear with me: Has anybody come up with a fitment index for helmet fitment generalities, or a standard as a basis of comparison? I realize manufacturers won't do this because it's not in their best interest, but there's some random internet knowledge of this if one has the time / patience to scour helmet forums.
For instance: I seem to have a "shoei" head and I loved my RF1200 but other shoeis like the GT-Air don't seem to fit me well. The Neotec 2 does, though - it's basically a modular RF1200 AFAICT.
I thought I read somewhere that the GT-Air fits a bit differently and might match "Arai heads" but I don't have an arai head, from what I've tried on.
A couple years ago I tried a Bell M9 helmet for dirt biking and it fit like a dream. After being not happy with the Shoei hornet x2 I sold it and switched to a Bell MX9 adventure helmet - and bam, fits much better. I really really wanted a Klim Krios but it just did not fit well. Oh well, saved a bunch of $$ there, haha.
The fitment within brands and models is difficult enough - I had to switch from a L rf1100 to an XL rf1200 shoei - let alone trying to switch between manufacturers. Also, it's impossible to find the exact brand, model, and size you want in a brick and mortar shop to try on exactly what you need - or a large vs. x large, etc:
I had to exercise my credit card with Revzilla a few years back when shopping for a Schuberth E1 by buying both a L and XL, until I found they just don't fit me and returned both.
Seems like it'd be interesting to have a matrix of "X helmet" and similar fitment types of different brands/models of helmets to that one to comparison shop / feature scope.