The amount of air space inside of your mask is referred to as the volume. A mask that is low volume has a small amount of air space. The amount of air space in the mask will depend on its design.
Single pane masks will give you a greater range of vision and a more unobstructed feel, but that is at the expense of more space inside. Double pain has less of a field of vision, but the space inside the mask is cut down.
A low-volume mask will make for better optics and will make it easier to clear the mask if it fills with water. The less water inside your mask, the easier it will be to clear.