We tend to underestimate the size of the world and overestimate (due to fear etc) the capability of humans.

The atomic bombs dropped on Japan were powerful FOR THEIR TIME. But it didn't even destroy a single (not very large) city. It certainly did not kill the entire population of that city, not even close. The immediate blast radius was something in the region of a few miles.

Even a bomb a thousand times more powerful (today) would not be capable of destroying a large city. Things work exponentially so we could perhaps get a blast radius of ten miles (depending on how it was exploded) with some of the larger hydrogen bombs of today.

The earth has a circumference of 25,000 miles (give or take). As a point of reference, a major volcanic eruption or earthquake releases far more energy than even our largest bombs.

These weapons are massively destructive but are no where close to where a few of them could wipe out a small region.