In reply to alps_p: Once upon a time, I could pack gear for a scottish winter trip of a couple of nights, including full rack, ropes, stove, tent, shared between two, and all the usual gear, about 500 spair pairs of socks, spare clothes and a lot of food, often fresh fruit and heavy stuff like that, into a 35l sack. I don't know how I did it but it would inevitably fit, after a lengthy ramming session.
Nowadays, I have a 100l pack, and use it a lot, packing is so easy. I take less gear than I used to, but, depending on the route, and if we'll be setting a base camp, I will take along a lot of unneccasary food and stuff, it might make me a little slower, but the increase in moral is worth it.
I have occasionally gone to the opposite extreme when not planning to bivy in alpine summer, but where it might happen, one small bag (25l) between two, one bivy bag shared in it, a jumper, map and compass, no sleeping bag, one sandwich each, maybe a bar of chocolate, a pack of emergency nuts, headtorch, reel of zinc oxide tape, a bandage, a pair of gloves and one mitts each, bottle of water, other guy just carries the rope and skeleton rack. Helmets are worn all the time.