Originally Posted by ObieX

Keep in mind a couple things:

1. There were quite a few basement floors to the towers for the debris to compact into and a lot of other areas underground. (parking floors etc).

2. A lot of the debris would have been pulverized and smashed into dust by the sheer force of the collapse. When you watch a video of the collapse you see a lot of "smoke" shooting out. Well, thats not smoke, thats pulverized concrete and other materials. Most of that ended up as a thick layer of dust over quite a large area.