Big packs over 40L are carefully engineered to assist the carrier to comfortably carry loads over longer distances. Especially important is how the designer adapt the ergonomics to how the body moves while walking- including shoulder and hip counter swing, spine flex under load, centre of gravity during activity, and the packs potential to exaggerate off-balance maneuvers. Women may find packs designed specifically for their frames are more comfortable, plus more energy efficient. Ultra lite backpacks reduce load - they also sacrifice comfort and ergonomics but if the load is light enough this trade off can be negligible. We recommend Arcteryx, Deuter, Marmot and Osprey.