Here we go then….
Full on l/w backpacking (tent, sleeping bag, mat, cooking gear, food, spare clothes, waterpoofs etc), you’ll need 45 – 50 litres at 9 – 12 kg. I know you can get lighter and more compact, but the above numbers aopply to my own gear winter (full 4 season bag/mat/tent) through to summer c/w two full days food. The Mountain Marathon brigade manage to get down to ~7kg and 25l but, in the middle of winter in a howling gale, camped high on your own, you’ll want more than their bare minimum.
I use a couple of climbing/mountaineering sacs (one 50 and the 55 litre). They are 1.3kg and 1.6kg respectively. You’ll save up to 0.5kg by going with the Golite Jam or OMM Villain.
To get everything in the Jam/Villain, you’ll need your other kit to be top notch and light. If it isn’t you’ll need more like 65 to 75 litres.
I’d buy the Villain if I didn’t already have a cupboard full of sacs 🙂