Just like clothing brands, different full face snorkel masks have different sizes as their standard. Masks are sized as XS, S, S/M, M/L, and XL.
It's not until you take a closer look at individual sizing charts, that you'll see small variations in general sizes.
Fit and Comfort doesn't come in all sizes
Masks made with higher quality materials have a softer, more flexible skirt. (The skirt is the piece that creates the seal between your face and the mask).
Some masks sitting at the lower end of the full face mask price range are made with stiffer materials that aren't so forgiving with faces of not-so-average size.
What all fail to mention is that it's not just the length of your face that counts for comfort and a good fit, but also it's width. The size of your nose also counts.