Loren Pechtel wrote:But look at the size difference between a missile and a 10-km asteroid.
Exactly. If you want to delve into the details, a KEW just like an asteroid has a potential energy that is released as kinetic energy when it hits something. The interesting part here is that the composition and the size determines how that potential energy is converted to different types of measurable energy.
Two projectiles that are vastly different in size and composition but with the same potential energy will release said energy differently.
An asteroid for example, will start converting large amounts of potential energy to heat when it enters an atmosphere - compare that to a KEW that is designed to reach its target with as little loss of energy as possible.
The size directly affects how the energy is distributed on impact. A sledgehammer hitting an iron-plate under a foot of water for example, creates a big splash and dents the plate while a bullet with the same potential energy creates a small splash and punches right through the plate. The former will most likely make you dripping wet while the latter most likely wont.