In a sense, a quilt, when it's closed tight beneath one, is like a bag which has only one vertical baffle, centered on the underside. I'm not sure how this works in practice, but it seems like the down under body weight, and movement, can squirt sideways, self-optimizing and avoiding compression. I do know that I sleep warmer in a quilt per ounce of carried weight than I do in a bag, and that the quilt seems to collapse around me better, leaving less air volume to heat, than with a bag.
One thing to think about is that bag companies have to design products that look good in a store--plump, with a pleasing shape. I think this moves the design of most bags in the direction of being a semi-rigid bottle, which makes it hard to get the bag as close to ones body as possible and eliminate intervening air. A good quilt just seems "floppier", and fits itself to ones body better.