Well, a little physics here!
Remember, that the more surface area of a bag you have, the less air goes through the bag at one area, thus, the bigger the bag, the less suction it will lose over a period of time. I love my Rainbow, but I feel as though it doesn't trap a lot of the dirt. I mean, with the water molecule being polar and wanting to attract statically charged objects, that is great, but I feel as though if I were to put a good piece of filter over the exhaust, I would see a lot of dirt. I will have to try that.
About the bag: The Compact seems to have a smaller bag than say, the Kirby Heritage. I love my Heritage. I also have an Air-Way canister (the new black one) as well as an Air-Way 88 Mk. II. All three are excellent vacuums. I don't like to use the Rainbow D-2 because of the problem of water in, water out... Good idea; very inefficient. Bags are DEFINITELY the way to go! Our new Air-Way has picked up so much dirt, it's just positively amazing to see it! The ENTIRE bag is full when we change, as the suction seems to be high enough to go through the pores that aren't clogged. My Kirby, since I don't vacuum much, is not very full, but the concept of a bag and a Sani-Emptor is the best idea ever! Small parts, like, for me, Legos, will fall to the Sani-Emptor instead of the bag. Then, the dust and small, lightweight things fly into the bag. Throw away the bag, empty the contents of the Emptor, and you are done! Very easy!
Kyle