The M and the SL-1 are the same: by the time you put the lenses on (anything other than the ONE pancake available for each system), the size advantages become increasingly negligible. By the time you have a couple of lenses and a flash, the size advantage is basically none. If you want to buy an EOS-M and use it EXCLUSIVELY with the 22 STM...sure, it's an an all right system (except for the AF, the lack of EVF, the crappy UI, etc)...if you buy the SL-1 and only use it with the 40 STM...I'd rather have the Fuji X100s. If you're going to use other lenses on it, I'd rather have a Rebel. Since those are your only options (use one lens only, or use other lenses), I really don't see the point of it.
While I tend to agree with your comments about size, I believe that is a similar "issue" among many of the micro 4/3 camera systems. They do tend to be "smaller" and "lighter" but they do not easily fit in pockets (though if you have a wife or gf, they do fit in her purse).
I also understand that the Fuji x100s is a great camera. Yet, look at this comparison:
The Fuji actually weighs a little more (albeit not much) and really isn't that much smaller. Yet, with the SL1, you have the ability to use any of your other ef or ef-s lenses On the EOS-M, I have left the 22 pancake on and it makes a great package when going out. Likewise, I will probably keep the 40 mm pancake on the SL1 most of the time. Yet, being able to take a 17-40/16-35 or even a t/s 24 as an additional lens really brings a lot more versatility than having just the fuji x100s (not to mention that the Fuji is substantially more expensive).
Now, as for just purchasing a Rebel, you may have a point.
However, I can tell you from personal experience and use (my daughter has a T3i which I have used with the 40 and I have used the 40 on my 5D3), the SL1 with the 40 is a much more compact and light system than either the T3i or the 5D3.
Until you have actually had a chance to take it out, shoot with it and see how you may or may not like it, it is very hard to "judge" a camera and whether it will "work" for you. There are a lot of great cameras on the market. Some people prefer one manufacturer over another based on how the camera feels or the the placement of the controls/buttons.
For me, I like it. Then again, I like the EOS-M too. I look at them and see "how" I can use them and if they will work for me and my situations. Can either the EOS-M or the SL1 match my 5D3? No, but they were never intended to either.