I went through the reviews and bought the medium one because Large seemed to large. I have a Schipperke with about 8/9Kg and it fits well with enough room to stay with his head out of the backpack. He loved.
The backpack is very good, the delay and the seams are even, for the dwarf poodle the size M is better. I bought S, a little too small, but everything suits me, I just wanted the muzzle to be outdoor.