Any down bag will compress a lot smaller than synthetic, and will last many times longer. Check bags from Montbell, Western Mountaineering, Feathered Friends or the Marmot Lithium. Pricey, but with proper care will last a lifetime. And they certainly much lighter than a comparably warm synthetic.

Your first priority in any sleeping system should be comfort. That's why you've been warned against trying to use a 20*F bag in 0*F temperatures. Second is weight. Compacted size should be a distant third.