i am an average build dude (72kg and just shy of 5'10") and have room under that M for a tee and a fleece. though couldn't go really bulky fleece but as normally only worn while doing outdoory stuff that isn't necessary. has an excellent warmth to it for its lightness.
How do they size up? Looking to wear over a tee and a fleece for hiking. That M looks small on a slim bloke.