The Express is my favorite smallish pack. It has a suspension so you can carry quite a bit of weight. If you use the waist belt, it alone can support the pack. It can accept up to 3 Dock-N-Lock external pockets, if you have to add extra junk to it.
Pictured there, you can see it's full with: ECWCS parka, fleece jacket, soft shell, rear beanbag (the blue sock looking thing), binocs, 100 rds 308, brass bag, camera, 2 liters of water, and two apples.
It carries the weight in a vertical stack, and keeps it close to your back.