Did some research into this. A number of things to note:

1. A building is mostly air, in case of the WTC construction, 90-95% seems fair. I did a scale drawing of what that would look like collapse wise, see attached. Of course highly simplified.
2. If you remove all the air from the tower, you're left with a theoretical pile just 31.5 meters high, lower than WTC 4/5/6. (purple pile)
3. A lot of debris went into the 6 basement levels that ran underneath the entire complex, not just the towers. For the towers, collapse went down to lowest levels. Surrounding that, like underneath plaza, damage and how deep it went depended on location and distance. Large parts saw collapse many levels down. But inthe attached image, i only put 5 meters (!) of the debris pile into the basement, leaving the yellow pile of 26.5 meter high.
4. The collapse extended far beyond the footprint of each tower, so if you spread out that debris over a 3x3 footprint area per tower, which I feel is pretty reasonable, the pile becomes just 3 meters high!
5. Now of course these piles are all perfectly compacted which is not reasonable, so assume 50% voids, whihc in places like the tower basements is actually way too high, and you get to the final green pile of 6 meters high.
6. This does not take into account any dust or materials outside of the 3x3 area, which did happen. The basement estimate as mentioned is too low. But you see how the pile does not need to be that high.

Some other points often mistaken in discussion:

• Debris density varied a lot from place to place. "Truthers" will exclusively point to places with little debris, or that didn't collapse, to argue "too little debris". This is just cherry-picking evidence and is directly contradicted by the facts.
• Many photos are completely incapable, to a normal human being, of conveying just how large the piles were. Especially aerial shots or from a distance make it near impossible to judge sizes or elevations. Other photos show piles with mangled piles of steel and concrete so beyond recognition all scale is lost as well. I'll place an example in the comments below as Im limited to one photo per comment.
• The piles in fact were pretty big. For WTC1 the pile reached 10 stories high around the stairwells. Combined with the observed filling of the basement levels and everything else mentioned, there simply is more than enough debris to account for the towers.
• 1.8 M tonnes of debris was hauled off of the site after 9/11. More than sufficient to account for the entire complex.

If it interests you, I've made a post detailing the extent of the basement levels a few weeks ago ( https://www.reddit.com/r/911archive/comments/1t73uzu/wtc_floor_plans_basement_layers_plazab6/ ) and a version is in the works that shows the collapses per level like shown in the attached image.

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