Just to show some size comparisons, here are some quick snapshots of these different cameras. First is the Leica M6 compared to the Pentax MX, both with 35mm f2.0 lenses of the time the cameras were made:
Notice the MX looks smaller from the front, and it feels smaller in your hands. However, the Leica with 35mm is a much thinner camera, as the lens doesn't need to be a retrofocus design to clear the mirror box:
Also for comparison, here is the Pentax LX and MX side-by side:
The LX grew a bit compared to the MX, but is still a great size to handle.
Pentax even started out as a small camera. Here's the Asahiflex compared to the MX:
The Asahiflex was a much earlier camera, of the same vintage as the Leica IIIc-IIIf series:
The Leica was then a faster handing camera than the Asahiflex, due to the waist-level finder on the Asahi. That advantage disappeared quickly, and the MX is a very fast-handling camera.