Faces that aren't so average
As we know, faces come in all shapes and sizes. What manufacturers are trying to do is fit a standard sized full face mask, to a variety of face shapes. If your face is wider than most, you'll probably find the mask is ok, but it may not feel as secure as you'd hoped.
Manufacturers claim the range of sizes will cover about 90% of the population, so there's a small chance you may not get a mask to fit comfortably.
If you've been blessed with a larger than normal sized nose, you could find that no matter which brand you buy, most full face snorkel masks will rub on the bridge of your nose. This may cause some minor discomfort. If the mask you choose is not generous in this area or the material is stiff and unbending, this can really cause some pain.