I am not sure if this falls into the category the OP's asking about, and not exactly ultralight...




This type of bags can serve very well as backpacks. They are designed specifically to replace the panniers, providing actually more space and they fit rear racks just as well as the conventional panniers do. Though there is so much space that is easy to overload your rear ruining the balance a bit. But it can be tighten down to rather small size.
For some offroad mountain trips people even use to add straps where they can attach their bicycles to, so that one can carry the luggage AND the bike at the same time.



This type of bags is more or less a standard in exUSSR area, just as the panniers are in the Western world.