DCWatch has posted an internal Canon presentation about the advantages of the RF mount’s large diameter. Canon describes why they developed a new mount over using the existing EF-M mount. The large diameter of the RF mount allows for simpler and better-balanced lens design.
The RF mount allows for the larger lens elements to be near the rear of the lens and closer to the camera mount and the image sensor. This should provide better balance when larger aperture lenses are affixed to the EOS R. While this could technically have been done with the EF-M mount, it’s far easier for Canon to design lenses for the RF mount.
Comparing EF and RF lens design...

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