Your layers of clothing for fluctuating temperatures during the day should fit in a medium-sized rucksack (2500–3500 cubic inches, or 40–55 liters). Although they nearly always depart earlier in the morning than you do, the porters and yaks who will transport your bulky duffel bags do not necessarily travel more quickly than you do. The majority of the things you bring every day are things you don't need to hike in, not things you need to be active with. For instance, the jacket and cap you put on in the morning before the sun comes out, the camera and notebook you use to take notes while taking breaks. Don't take the advice to pack lighter so you can carry less. The weight is well distributed, and it is much simpler to pack in slightly larger packs (particularly with chilly fingers!).