[pct-l] Ultra-compressible down sleeping bags?

[Rachael Howard]

So I finally found a backpack that can carry 30lbs of gear without killing
my back and shoulders.  I ended up with the Osprey Aura 50, and I love it.
However, I'm worried that my gear may not fit in it.  The biggest items I
have are my Contrail tarptent, which I refuse to part with, and my sleeping
bag.  Right now I'm using an old green synthetic Army sleeping bag for my
1-2 night camping trips.  Even in the compression bag, it takes up about 1/3
of my interior backpack space.  My question is thus:  what size does the
average ultralight down mummy bag, like the Nunatak Alpinist or the Western
Mountaineering Ultralight Super, compress down to?  Just want to make sure
I'll have enough room in this pack before I break it in.

Thanks,
Rachael