Thanks for clearing that up, I'm not sure how I missed the different sizes. As to why I am looking to swap, as good as the 5D3 is, its also a little on the bulky side for an everyday carry around. Originally I was looking at getting the M5 as a carry around along with the efm/ef adaptor. However I like the look and DOF that the FF gives, after seeing an offer on the A7ii, it works out as a straight swap £ for £. The money I was going to spend on the M5 will go on a 7Dii (don't tell the wife). This way I have a smaller carry around camera which still has the look and low light ability of FF, along with a fast focusing/high fps 7Dii for wildlife and motorsports.
I am however happy to listen to other suggestions, is the A7ii that bad compare to the 5D3?