It's all to do with momentum - mass x velocity.
160 tonnes at 800km/hr is a fair whack of momentum.
Flight 77 struck the west side of the Pentagon at the 1st Floor level just inside Wedge 1 near the 4th Corridor and proceeded diagonally at an 60 approximate 42 deg angle toward the 5th Corridor in the mostly vacant Wedge 2.
After the nose of the plane hit the Pentagon a huge fireball burst upward and rose 200 feet above the roof.
Multiple explosions occurred as the plane smashed through the building.
The front part of the relatively weak fuselage disintegrated, but the mid-section and tail-end continued moving for another fraction of a second, progressively destroying segments of the building further inward.
The chain of destruction resulted in parts of the plane ending up inside the Pentagon in reverse of the order they had entered it, with the tail-end of the airliner penetrating the greatest distance into the building.
Remarkably, these circumstances meant that the bodies of the passengers in the rear of the air~ craft travelled deeper into the ground floor of the building than did those in the front. The largest concentration of body parts was found at the deepest area of penetration-the C Ring.23
While the immediate building interior area hit by the nose of the aircraft was small, the section subsequently wrecked by the large plane debris and multiple explosions of jet fuel from the ruptured tanks seconds after impact spanned a larger irregular area of more than an acre on each of the 1st and 2nd Floors.
