@Eris it'd designed fairly big in the first place. as an XL helmet wearer (I've got a weird skull), though, I usually have no problem with this base design.

More technically speaking, there's about 5.5 inches of curvature from between the eyes to where the mask flattens out to horizontal on the top of the head, and as you can see in the pic, most of that distance is vertical. In short, it works for a very large majority of heads.