This has been treated over in the Leica M-D threads, but the bottom line is that the thickness of the digital M body can only vary a couple of millimeters at most due to the lens/lens mount geometry, the rangefinder mechanism, and the thickness of the body casting and sensor assembly.
The LCD hangs off the back of the body and barely adds more than 2mm to its thickness when present (basically, half its thickness is the thickness of the body's leatherette covering). It also weighs less than 10 grams.
Unless Leica chooses to radically reengineer the rangefinder, puts the lens mount on a pedestal, and sensor assemblies become paper thin, the M is likely going to stay pretty much the way it is with respect to size and weight. No problem for me: I like the feel of the typ 240/246/262 models even more than my M4-2, which feels a bit thin and cramped by comparison.