The volume of air inside the mask, between the face and the lens, represents the internal volume.
For free-divers and spearfishers, the internal volume of the mask must be as small as possible to facilitate equalisation.
The mask gets pressed against the face when diving down. As you dive, pressure gets applied to the mask's flexible skirt, reducing the internal volume and pressing the mask against the face.
This phenomenon can be avoided by equalising the internal pressure of the mask and the external pressure exerted by the water. Simply breathe out gently through your nose. Air is very valuable in underwater sports, so this equalising effect must be kept to a minimum.