My example was just an exageration, I apologize for not giving a more logical senario, same goes with an atomic bomb the size of a pen. What I was trying to say with the pen sized atomic bomb is that an atomic bomb is more destructive than convention explosives.
Here's another example: You have a briefcase full of dynamite, you place it within an abandoned city on a city block. You detonate it, what happens? A few buildings get damaged. If you had a briefcase with an atomic bomb in it, and do the same as you did with the briefcase full of dynamite, what would happen? The city block (and probably a few more city blocks) would be demolished.
The thing about the meteor: Meteors enter the Earth's atmosphere travelling close to the speed of sound (I don't know the exact speed), that's enough to leave a huge creator. If you had a meteor that was travel at half the speed of light (which is extremely faster than the speed of sound) then probably the Earth would get demolished, or severely damaged, or just have a huge creator in it.