Depends on how much you gotta carry. I use a small backpack almost all the time. I just don't have that much to carry. If OP is carrying a lot of stuff, then certainly a larger backpack would be needed, but there is a point where carrying a heavy load on your back is going to not only put pressure on your back, but also your hands, and worst of all, your sit bone. I've carry a 15-lb watermelon one time on a backpack, and it was just for 2 km, and I felt it most on my sit bone.

Might want to reconsider this idea, OP.