First off, welcome to LVL!
The digital M cameras, starting with the M8 - are all noticeably thicker feeling in hand than the film cameras that preceded them. Visually, it's not so obvious. Just nature of the beast... Between the shutter, sensor, LCD... It all adds up.
If the body and top/bottom plates were made of cheaper materials, people would complain about that instead. The bodies have (largely) had a history of using brass for the top and bottom plates and anything less probably wouldn't be well received. Could Leica make a lightweight digital M? Absolutely. Go for broke and use composite materials for the body, top and bottom. Though I fear the price...
It's all relative. Pick up a Canon 5D Mark III and comparable L lens - and then pick up the Leica.
Of course, you could also check out the Fuji X series of cameras if you really need the light weight (e.g. X100T or X-Pro1). Granted, not full frame - but that's okay. You can adapter your M lenses to the latter.