The 10-18 seems like it might be a good match (if you already have an EF/EF-S ->M adapter). Getting good reviews and pretty small. Any reason not to go this route?
I just got a 6D, kept my 60D as a backup/beater camera, so the M is relegated to times I'm bringing some sort of backpack or shoulder bag while playing tourist but not so much that I'm bringing one of the bigger cameras. Debating whether to invest in a wide-angle (trip to Italy coming up), or pare back even more and spend the money on something like an RX100iii that takes great photos but is more limited in focal length.
The 11-22 is significantly smaller than the 10-18 + adapter.
I had been considering the RX100 III, since I used to bring along a PowerShot S95/100. However...in bright light, small sensors do fine (nearly as good as larger sensors if you don't need shallow DoF), but in dim light you want the biggest sensor you can carry. That's what I like about the M – relatively larger sensor in a small package with lens flexibility. The 11-22 and 55-200 together aren't much more outlay than the RX100 III. The M + 22/2 does decently in low light (not nearly as good as my 1D X + 35/1.4L, though).
I find the M + 18-55 carries nicely in a Lowepro Dashpoint 30 on my belt. I have the 22/2 pancake in a Lowepro Lens Case 8x6 in the backpack that I bring (I have 3 kids, one 15-months, so I need snacks, diapers, water, raincoat/sweater or towel, etc., on pretty much every outing). The 8x6 case holds the 18-55 when I swap. The other two M lenses would be easy enough to carry in separate lens cases, and the whole kit would take less room than the 1D X + 24-70/2.8 II even in a neoprene cover in the backpack.