The GMH is a bit smaller than other available helmets like the FPH2 helmet that many people use today.
The size difference is not extremely noticable, but side by side, the differences get a little bit clearer.
Some reading material about that helmet:
The Grant McCune Boba Fett Helmet
I've also made a thread in which I compare a Bobamaker V4 and a GMH:
Bobamaker V4 Helmet Kit