While it's easy to annihilate a city, it's hard to destroy a bunker or missile silo. The people who keep nuclear weapons want to be able to hit a narrow target

While it's easy to annihilate a city, it's hard to destroy a bunker or missile silo. The people who keep nuclear weapons want to be able to hit a narrow target while evading countermeasures. Maybe even burrow into the ground before setting off the main charge. This is easier with smaller payloads. There was once a wide consensus to not continue with this arms race. But unfortunately it broke down.