You have to understand that the Pentagon is puposefully hardened against attack. It has very thick steel-reinforced concrete walls, blast windows and is built like the proverbial brick shithouse. The Towers could never have been built like taht and still stood as tall as they did. Steel-frame skyscrapers are pretty open structures, under the cladding. The pentagon was designed to survive nearly any conceivable conventional bombing attack. This is why so little was left of the plane, and why the damage appeared to be much less than that at the Towers.