Steve Gandy claims that there's nothing more compact than a Leica M body with a collapsible lens. And he has some pics to prove it!
I've seen and handled (a bit only) the Leica III. It feels incredibly complex, delicate yet tough. Hard to describe. The viewfinder turned me off, though. Later, when I looked through the VF of a M3 I lost my soul to the Leica devil... 🙁
Which, of course, joined forces with the Canonet angel and evicted the mischievous Nikon SLR spirit, which had survived in my soul long enough, crouching in a corner, all squashed up! 😀