I strongly suggest you try it on for an extended time before it's too late to return it.
I originally purchased the matte black G9 in medium because it fit perfectly in the store. There was room for maybe 2 clicks of the adjuster. After wearing it for a couple hours I had to return it because it made my ears hurt like crazy. The pressure on the ears was too much for me. I got a large sized G9 and it definitely feels looser on my head, and I look goofier with the bigger helmet, but my ears dont hurt. Part of the problem is that I'm right on the border between M and L with a 59cm noggin.
Fits perfectly with my Oakley A-Frame goggles. No gap at all.