Griff's right about lj's advice. Stuff it.
Also about the larger sack. You may well consider a compression sack. If you're shaving ounces (and that's a worthy goal) you would do well to consider one made from silnylon. This material is silicone impregnated ripstop nylon. Very lightweight, tough and thanks to the silicone impregnation, both slick and water resistant. The seams are typically not sealed, therefore not water-tight. I'm happy with the three or four I use all the time camping. You'll like it too. If you're a brand-name kinda person, here's are a couple of links that are excellent, but don't get stuck on the name, just the structure of how it's made.
Granite Gear
Well, I can't find the other links right now. so. look for compression sacks to save space, if they're silnylon, you get a waterresitance bonus. But I think you just need to practice stuffing it in there. One tip, start compressing it firmly in the regular sack early in the process, not just at the end when you have a big mushroom cap of sleeping bag to get into the last inch of sack. Start mashing early. If it's too tight, get a bigger sack.