Peter, are you sure the 12 Fret "S" Dreadnoughts have a greater body volume than the 14 fret "D"'s" ?
I used to own a D28S many years ago, and I seem to remember the length/width/depth dimensions of the body were pretty much the same as a regular 14 fret Dread. But, I could be wrong ...
I threw out my collection of Martin flyers, which were out of date, but Elderly prints Martin size stats in its catalog.
They give the 14-fret dread as being 40 1/2" long overall, and the body length as 20." The 12-fretter is given out to be 39 9/16" long overall, but the body length is 21."
So the 14-fret is almost an inch longer overall, but an inch shorter in the body, because of the more rounded shoulders. Since the width is the same, the 12-fret should have somewhat more total volume.