I, too, pondered this scene quite extensively. I'm guessing the best answer you could wrestle out of IW would be something along the lines of...
Infinity Ward Designer said:"Well, uh... see, explosions have mass... and that spreads really fast... and... uh... WHAT PART OF SEMI-REALISTIC MILITARY SHOOTER DON'T YOU UNDERSTAND?! IT LOOKED COOL, SO WE DID IT! THERE! WE MADE YOU INTO AN ASTRONAUT THAT GOT BLOWN UP BY A NUCLEAR MISSILE, HOW MUCH MORE AWESOME DO WE HAVE TO BE BEFORE YOU STOP CARING ABOUT THE DETAILS?!"