for multi-day trips you really need a huge pack, imho. i use the gregory massif (6400+ cubic inches) for winter or expedition trips lasting around 5 days, after that i also haul a sled. when you think about how much more volume your winter gear has you really need the space, unless you don't mind strapping a bunch of stuff to the outside of the pack. then you look like a yard sale and you have to be concerned with loosing stuff.
if carrying a big pack isn't to your liking then you really need to get a good, minimalistic system dialed in, it also doesn't hurt if you don't mind a little suffering. this means the right boots, base layer, action layer (softshells), shell layer (goretex), warmth layer (puffy), and glove/hat layer. a really warm sleeping bag (-20 +) is heavy. bring along your +5/+10 bag and wear puffy to bed and lay on your other clothes in the bag, this makes a big difference. do you really need that xtra base layer to sleep in, no. do you really need more then 2 pair of socks, no. drop the tent in favor of a bivy sack, if you can't do that then split it up between the group. that goes for the food as well. but in the end you can only take this so far.
hopefully this was helpful.