You are mistaken. The eos-m mount can physically not handle an ff image circle. Sony was smarzer and made their e-mount just barely large enough to also handle ff. Canon was stupid, as so often. Aps-c only.
Can you explain why this is not possible? The flange distance is 18mm for both Sony E-mount and EF-M mount and they have approximately the same diameter.
It was discussed here and elsewhere many times over. Just scroll down on the dpreview article:
While the ef-m mount has a huge 58mm outrr diamezer it has a freakin narrow throat. So small, that neither canon is ashamed to provide the specs. It must be a couple mm less than the sony e-mount which has a clearance of 46.1mm. Ef-m is definitely not ff-capable.
A HUGE mistake by canon. They did not believe, sony cokld come up with a small bodied, hi-spec FF-sensored a7/R. And got burned.