I'd definitely look for a cheap (barrel) synthetic bag to use over the MtnHardwear bag. If you buy/make a compression sack you can get the volume down quite a bit.
Fleece doesn't compress much.
Wearing your down jacket/down vest for sleeping will help, as will a really warm hat.
Using a vapor barrier system can make a huge difference in winter, but that's another topic. Probably the most 'bang for the buck'.....
IMO, if you are planning on winter camping, you should get a large pack so you can take plenty of insulation, extra food & fuel, etc. Things can get nasty in a hurry in winter.