Here's the link, again, to the visual showing how big a Boeing 757 would be compared to the Pentagon. In the video, the projectile doesn't come anywhere near as high up on the building as a Boeing 757 would.

http://www.davesweb.cnchost.com/pentagon&plane.jpeg


Plus, if a Boeing 757 had hit the building, either there would have been wing-shaped holes in the building, or there would have been wings or wing wreckage on the lawn outside the building. Eyewitnesses have stated that they did not see any such wreckage or wing holes immediately after the initial blast. And in the above linked picture, there is no corresponding wing-related damage at the hight that the wings would be had they hit the building.