In reply to Paul Saunders:
A Single small sack is cheaper than a big sack. I use one sack, if it doesn't fit in that I empty the sack and look at what I can leave behind. Not interested in lighter stuff, just less stuff.
1) You can pack all your kit away from light-fingered folk when in a hut.
Less stuff means less to nick.
2) You can easily stuff a belay jacket in without compressing it.
A what?!!!! Get some blubber on you lad!
3) You can fit the rope in and keep it dry if neccessary.
I can do that with a 30L.
4) I find a thin large profile (such as you get with a compressed larger sac) doesn't overbalance me as much as a more blocky smaller sac.
No argument with that.
5) When rummaging around on a belay just enlarge the sac rather than take things out to get to the bottom... less chance of dropping stuff.
Pack it correctly so that you can get to stuff.
6) I've yet to find a 30l sac with gear loops/axe-holsters etc.
There used to be a Lowe one but I don't think they make it anymore. If you are on a glacier and you wish to stuff the axe away slide it down between your back and the sack.
7) I can fit big boots in the sac when changing to rock boots (not a fan of approach shoes in the alps personally).
Personal choice, I'd wear Walsh fell running shoes if I felt I could get away with it: getting to west face of Blaitiere for example.
8) I can over-pack water and bin it if neccessary, without compromising space.
Get a stronger stomach and a short length of plastic tubing then you can sup water from pools and puddles.
Seriously though, I have been asked both in real life and on the web about what to take for X, Y or Z, only to be told "Oh but I need to take this, that and the other". To which my reply is "you don't need that". Later when I ask if they succeeded it's usually a negative reply and it turns out that they were too tired from carrying "this, that and t'other". Answer? Don't take it! And if you are not taking much kit then you do not need a big sack. QED.
Bob