Another way of saying essentially the same thing is that a circle offers the maximum cross-sectional area for a given length around the perimeter. Thus, a cylinder of the same volume will generally require less material (and cost).
Actually, that is an additional reason for it being lighter. With the curved side, it is stronger than a flat side, if the material is the same. With a flat side, there is a tendency for it to be weaker in the middle than near the corners. (To counteract that tendency, it is common in high quality boxes to have internal bracing, which adds weight in itself, and also takes up space inside the box, requiring the box to be larger in order to maintain the same internal volume that it would have had without the bracing. The box being larger, of course, makes the box weigh more, too.) But a curved surface does not have that problem. Consequently, a cylinder is able to have thinner walls and/or less strong material and still have the side be equally strong and equally immune to flexing. But you are right, even if the material were exactly the same, the cylinder would use less material for the same internal volume, which is another reason why cylinder subwoofers weigh less than equivalent box subwoofers.
If one compares the SVS cylinders with their own comparable box subwoofers, the difference in weight is fairly dramatic. Anyone who does not require the box shape should seriously consider a cylinder.
