Question : Question In the movie The Avengers 2: Age of Ultron, the : 763514

Question

In the movie The Avengers 2: Age of Ultron, the Earth is threatened by a floating city in the sky that Ultron wants to drop from maximum height. The surrounding area is saved when the city starts crumbling apart and the rubble falls down on the ground below. Why would having the city crumble into rubble and fall be less powerful than dropping the whole object? Either way the same amount of mass falls from the same height but I think they would have similar Kinetic Energies unless the air friction led to a huge amount of disintegration of the falling rubble in the atmosphere or really changed the terminal velocity of the falling pieces. I think this is unlikely. So is it because of the pressure from such a large object impacting the Earth, creating a large impact crater? I'm confused as to how the physics would work out.