Notice how the radius of destruction is increased tenfold, despite its yield increased a million fold? That's the realistic limiting factor in the size of nuclear weapons, their effectiveness drops off rapidly.
Anything you would want to target is within a 1km vertical range across the surface of Earth. When you have a sphere of absolute destruction with a 120km diameter, the VAST majority of that energy goes into space.
This is why we have MIRVs, because in your case you could make a million megaton nukes and carpet a massive stretch of land (about a fifth of Earth's total surface area, or the every square inch of the continents excluding Antarctica) for the same yield.