EF-M lenses are still somewhat smaller than EF or EF-S lenses making the entire thing smaller.
There might be some new lenses coming as well. Canonrumors points out that the promotional video shows a f/1.2 setting !! I am hoping for a 50/1.2 prime
http://www.canonrumors.com/2015/02/an-ef-m-f1-2-lens/
They are quite a bit smaller. The biggest difference is, however, the body size. SL1 is narrower than a Rebel but a thick brick nevertheless, so I don't see much advantage over a Rebel - and it's substantially more cramped. The M (or any other similarly sized mirrorless) is tiny by comparison. Still not "pocketable" in a sense that it won't fit in my jeans, but I can stick my camera with a 16/2.8 pancake or 30/2.8 prime in my coat pocket easily.
I hope Canon releases more lenses and prices them reasonably, I like to have alternatives (although now I am somewhat happy with Sony).





