I wrestled with this a while back. This issue I found is if you think about bag weight you could divide it into the fill and everything else. The fill goes up or down depending on the rating and that impacts weight. The everything else also goes up for more stout feature rich bags - think draft tubes and collars, but there is a floor for this piece. Sure you can save weight by cutting features and using lightweight material but you probably end up with 3/4 to 1 pound without fill and then another 1/2 to 3/4 of pound of fill for a fairly lightly rated bag. There are exceptions of course. Using down is a way to cut weight, but I like down less for warm weather when I am more likely to get rain than snow. There are also ultra lite quilts and such and they have their charms but you give up some function.
I had a kifaru woobie and a long love affair with poncho liners but wanted something with a zipper. The woobie is great by the way and bombproof but weighs 2#.
so after poking around a bit i Found some really light but also pricey down options Like the western mountaineering everlite for about a pound. Zpacks and some of the light weight gear makers had bags also in the weight and price range.
reading the reviews had me a bit concerned about durability. I wanted something that could be a warmweather bag and a glassing or camp blanket.
i looked hard at a few options and ended up with a snug pack jungle bag - which is cheap and bombproof but close to 2#. I also bought a helicon swagman roll which is a poncho liner that zips to a bag and can be worn with a hood. It is lighter at about 1.5#. Both are great and function as lightweight bags plus wearable blankets. Paired with a borah bivy I have slept well in the 40s With minimal clothing And in the 80s using them as a quilt.
I could save 1/2 to 3/4 of a pound going with an ultralight quilt but I like the durability and better weather resistance of the synthetic. I have used the jungle bag or the swagman roll from chilly summer nights in the sierras and Grand Canyon to a boat blanket and Florida swamps without having to feel like I need to baby them. Partly because of price and partly because of confidence in the materials, I do t think anything about Throwing them down as a shooting mat, ground blanket, you name it.