Also, I have what I find to be a pretty satisfactory answer to one of my verbal querries: Pithy. The definition that interests me particularly is "Full or composed of pith." My understanding of pith is that it is bulky, and not necessarily dense. Pith is made for moving water through tiny holes and is thus full of air when dried. Pithy materials (wood, plants) tend to float in water, meaning that they have a lower density than water. Thus, I postulate that a good opposite for "dense" is "pithy."