There's also scale issues at play - as your bomb gets larger and larger, relatively more of its energy escapes into space and isn't directed into the ground.
The link in question analyzes the possibility of creating a doomsday weapon that could launch a projectile that would destroy all life on another earth sized planet remotely. That is a far more difficult task than just destroying life on the planet the bomb is on. The limiting factor in question is also fission materials to serve as triggers for the primary explosion in each of hundreds of thousands of shaped thermonuclear explosions. The massive numbers... (read more)
http://neo.jpl.nasa.gov/risk/2012da14.html
http://rt.com/news/paint-asteroid-earth-nasa-767/
Seems like a good opportunity to bring up existential risks. And A friendly reminder that NASA is in fact pretty damned important.
Thoughts?