I tend to pack pretty lightly when it comes to bedding if I am by my self, as I wear a microfleece combination like the Platatac Half-zip jacket and the matching microfleece sniper pants which keep me pretty toasty, and a light sleeping bag like my Aurora Wanderer, and maybe the Tribe Provisions Go-Anywhere woobie for comfort. With company, I usually zip two of the Wanderers together, or make a nest of combinations of blankets and bags.
However, when it gets really cold, and you're on your lonesome in the bush there is certainly room for a good high-loft sleeping bag in everyones kit.
I have a fairly dated Therm-a-Rest brand sleeping bag, so old it's no longer listed and I cant remember what the ID of it is, but the bag is BIG, and very warm.
Filled with Polarguard HV - high void continuous filament, which is a durable synthetic insulation made from hollow, uncut polyester filaments, it maintains high loft even when wet and is apparently 25% lighter than standard Polarguard.
Fast drying, moisture resistant yet machine washable, it's a really good filling, and I haven't noticed it shifting, clumping or having any of those cold-spots I recall from childhood back-yard sleeping-bags.
It's also considered highly compressible, as well as being odor, mildew, fungus and allergen free.
Mind you, it's an old model, and I have no doubt more modern bags, with modern fillings blow this out of the water.
Here it is beside my lightweight Aurora Wanderer Superdown sleeping bag. The Therm-a-Rest bag is standard rectangular in shape, 86cm (34") x 193cm (76") but feels even longer. I'm 6'4" and you can see it stretching higher than me. The Wanderer is a more modest 75cm (30")x 180cm (63") but it has a hood, in the mummy-style, to keep me in, even so I often find myself popped out, and cold-shouldered on chilly nights.
The Therm-a-Rest bag has a 100% nylon outer shell, and a 65% polyester 35% cotton liner for breathablity. It is rated down to -5oC (20oF) where the Wanderer is only a 0oc (30oF) bag. The difference in size and bulk of the fill however, make that difference noticeable.
Both have tape-covered, double ended zippers, to shut out drafts as well as giving you feet-openings. ((Don't wear boots to bed people!)).
At 1.2kg (2lbs 10oz) it isn't that much heavier than the 900g (just shy of 2lbs), there is not much difference in the load when hiking, but the difference is in bulk.
The Therm-a-Rest packs down to a considerable 85cm (34") x 105cm (41") bundle where as the Wanderer only measures 51cm (21") x 61cm (24"), MUCH smaller and that equates to easier carriage, but at the expense of cold-weather comfort ... You'd have to make the call, how cold is it going to get, and how much do you need to lug around.
I look forwards to upgrading at some stage to an even more modern, hopefully more compressible sleeping bag, but for now I have options.