I do have a T3i with 5 len's and a battery pack, so basically anything smaller than that will be good for me.
I originally was very interested in the M but the more I look at it I don't think its for me ( unless I can find a used one cheap ).
The reason I bought an M instead of a Sony, Olympus, Fuji or Nikon mirrorless camera was computability with my EF lenses via a Canon adapter. I normally use mine with the EF-M 22/2 lens, but occasionally use my 50 1.4 (portraits) or 70-200 (added reach over my full frame 6D). New M prices are still really good vs. competing brands as well as you can get kits for $349 or $399 from B&H and others.
Had I been just looking at mirrorless camera systems independent of my Canon DSLR equipment, I may have made a different choice. But, given the compatability, features and price, the EOS-M was the best option for me.