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OP, not everything can fold down to the size of a book of matches, and a subzero sleeping bag stuffed with an entire flock of geese is one of those things.
Put it in one of those vacuum storage bags, then vacuum out all the air. That is the best-case scenario for how much its bulk can be reduced under ideal circumstances. Now take it out again, because you won't have a vacuum and an electrical outlet handy on the trail.
You have two options: Stuff it into the tightest stuffsack that will fit it and put it in (or lash it onto) a backpack that can hold it, or get a less bulky sleeping bag. That's it.
Did you want us to invent a shrink-ray for you, you stupid motherfucker? A pocket-sized alternate dimension that you can teleport it into and out of as needed? What sort of magical advice are you expecting, here?
If you were to, say, kill yourself, that would technically solve the problem, as you would no longer have any need of a sleeping bag, nor care whether or not said bag is too bulky.
