Completely right. The major military uses of nuclear weapons are as sources of powerful shockwaves which are devastating to drag-sensitive targets, and as fires

Completely right. The major military uses of nuclear weapons are as sources of powerful shockwaves which are devastating to drag-sensitive targets, and as firestarters. In both cases, you can optimize for these effects over a given area by using 100kt-1mt warheads, and airbursts with overlapping blast radii. The problem is that as yield rises, the losses to the upper atmosphere are proportionally greater, as is fallout. Multiple targeting with smaller warheads solves that as well, which critically allows for more efficient burning of nuclear fuel. Finally, the larger the fireball, the more likely you’ll have it touching the the ground leading to losses, and kicking up more debris which will mix with fission products and unburned fuel. Of course that’s also one way to use a standard nuclear weapon in a manner more consistent with an enhanced radiation weapon, real “salting the earth” stuff. That is generally comsidered to be bad form, even among nuclear powers.