Asteroid Question???????

You always here Scientist say that this or that asteroid would create the same damage as 1000 nuclear bombs and so forth, Where do they get this information from? Isn't all an asteroid is made of is Iron and Rock? When it enters our atmosphere i know it lights on fire, I always thought it was just flatten the area it landed on, I was just wondering if anyone knew if there is something I'm missing is there anything in an asteroid that makes it so explosive the way scientist describe it?