This topic deserves much more time than I can give it right now.  However, I like this comparison by dpreview, so I include it here for my reference and you to use.  Sensor Sizes.

The picture makes it easier to see the difference between a 16mp 1/2.33, 4/3, APS-C and Full Frame.  Wow!  Same number of pixels, spread further apart.  Thus, the reason a compact camera with a small sensor shows more noise than one with a larger sensor.

I will be posting more about the noise as it relates to the Leica X series verses the M-8.  My wish list includes an M-Monochrome, but that will have to wait until they are used!