Have a look at the
Deuter Race EXP Air. It is small and narrow. The best feature is the Air Mesh Back Comfort System. This really works well. My wife used this for our two day jaunt in Japan. I used a slightly larger pack with the same type of mesh system and had no complaints.
The reason I like it on my back vs the seat bag is I want the bike to ride like it always rides. Swinging the bike back and forth as you climb a mountain isn't affected by all that weight that makes the bike top end heavy. On my back the weight is where I want it. Of course this works for me and as you read above others prefer something else.