The following may be obvious to some of you but I figured this out a little while back checking out frameless masks. We're always looking for low volume without sacrificing field of view. One way of doing that is by bringing the lens closer to the face. A frameless mask achieves this well but there's a limit to how close the lens can get. Once the lens is touching the ridge of your eyebrows that's it. At that point to further increase the sideways field of view it becomes necessary to make the lens wider so the mask becomes wider. Once it becomes wider you're starting to increase internal volume again.
The other way of increasing sideways field of view would be a rounded lens that curves around the eyes. But this would be much more expensive to manufacture than flat little pieces of tempered glass, I don't think anyone has done it. I then thought that a curved lens mask would be the ultimate mask when it occurred to me that a similar effect is achieved by separating the lens into two parts and angling them away from the middle mimicking a curve. Now you've gained sideways field of view without increasing volume.