Also look at the Freight Baggage backpacks. I have the Medium and have easily fit 40 pounds of groceries in it without fully expanding the bottom of the bag. They started coming with a different compression system (two on the side instead of only one) just after I bought my bag. I am actually considering the small bag because I rarely need the compacity of the medium.
I wish PAC made a backpack. That thing would probably be amazing.