If you're going to push the sleeping bag into the bottom of the pack without a waterproof bag around it, you need a waterproof pack liner. 2 mil plastic bags such as trash compactor bags (if you can find unscented) or contractor trash bags are the least expensive form of pack liner. Check for holes daily and use your duct tape if you find one.

Pack covers do not protect your pack contents in case of immersion (even the best of us have slipped during a dicey stream ford) or in a heavy rain in which water runs down your back and soaks into the part of the pack against your back. Stuff sacks are not waterproof because of their closure. I personally use lightweight dry bags for my sleeping bag and for my insulating clothing. They worked just fine the time I slipped and fell in even though I emptied several inches of water out of my pack afterwards.

I've been tempted to go back to the pack liner and try just stuffing the sleeping bag in the bottom. It would conform to the pack and the stuff on top of it much better than a hard lump inside a stuff sack or dry bag! I think that's why it seems to take up less space!

With a synthetic bag, though, about your only recourse is a compression sack. Otherwise, the bag will take up 3/4 of your pack! Unfortunately that's one reason synthetic bags don't last as long as down--they lose their ability to recover from being squashed.