I own both the FM2 and the leica M7. As well as other's.
Im still trying to get my head around your reasoning, which in mind is flawed. First off the FM2 isnt bigger than a rangefinder.
If you use a Nikkor 50 1.8 AIS on the FM2 I hardly think it's to heavy to carry around. I hear this arguemt and a lot on this forum and frankly it's not true. A SLR like a FM2 with a 50 1.8 AIS is just about the same size as a rangefinder with a 50 2.0
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Taken with Rollei twin lens on Tri-X
Nikkor 24 2.0 AIS on F2AS on Tri-X
Nikkor 135 2.0 AIS on F2AS Tri-X
Nikkor 135 2.0 AIS on F2AS on Tr-X
Nikkor 24 1.4 AFS on D3