Canon EF-M mount might possibly be "force-fit" for FF [similar to Sony E-mount, involving major design compromises], but it was definitely developed for APS-C only and not with FF in mind. At least that's what the Canon people in charge of EOS M / EF-M said.
- Where is the most important factor in developing a new mount called "EF - M"?
Nakamura: It is a mount diameter that satisfies the APS - C size image circle properly, but also to make it possible to realize small size and light weight. In addition, the operation feeling such as attaching and detaching the lens is designed to be the same as the EF mount.
- EF-M mount. The mount diameter is also reduced from the EF mount
Can the EF-M mount correspond to sensors larger than APS - C size, eg 35 mm full size sensor?
Kikuchi: I think that I can not do that. I can not say that if I do something strange, I will not physically enter ... .... It is a level that the amount of marginal light falls considerably or that you do not know what the image will look like.
http://dc.watch.impress.co.jp/docs/news/20120810_551672.html