I always thought the Gascan fit a little tight on me, until I tried on a pair with Matte frames today (Matte Black/Grey iridium). They seemed to be a fair bit looser and fit me much better. Is this something I'm just imagining since the Matte sticks to the skin less, or is the fit really a little different?

I'm not sure if it's necessarily based on color, and more just production variation. I've experienced the same thing with two Oil Drums. My Cinder Red ones were fairly loose on me and these Matte Brown ones I just got are quite a bit tighter, almost to the point of being uncomfortable.

that is true in my pairs also. My matte black / ruby fit way looser than my grape transparent frames, my rust, or any other polished finish

My crystal Gascans fit me nice, BUT could be tighter.
However, my Gray Gascans (matte finish) fit me more snug.

I did however once try switching lenses out of a pair of Grape Gascans to find out that swapping the lenses DID change the over all fit to a more snug fit than before.

Its as if, the lenses had more of a curvature to them for the frames to fit like that.

Matte frames slip less due to the texture.

K