Agreed on what you're saying about the space. But the saving I'm referring to is what is gained by either not having to place the cam over the valve stems/springs, nor having to utilize traditional rockers between a cam and a valve. Desmo seems more compact to me, even if only by a little bit.
Looking at that BMW head, it later occurred to me that it looks like the cam bearings may be smaller than Ducati cams, thus lowering the overall height of the head and valve cover slightly compared to a Duc. All my time was spent on 2V rubber band motors, though. I've never stuck my paws into a bevel head. Drooled on quite a few at the annual Daytona Ducati Days though.