Something to remember about larger bags/packs. They carry more and we generally tend to pack more if there is space. Hence the bags get HEAVY. I have the Tamarac Expedition 7. I carry a long lens mounted to a gripped 7D, 5DII body, and multiple lens, filters, cleaning stuff, other extraneous stuff. My bag typically weighs about 45 lbs which I believe is approximately 20.4kg
It is not too bad for a backpack, but way too heavy for a messenger bag. Also if you are hiking on uneven territory- which is most likely - it can throw your balance off if you are not careful.
I did buy a large messenger bag and used it one day. I now walk leaning to one side...