[korethr]

If you read the linked book, it covers this point. Yes, the tech exists to build bombs with multi-megaton yields and has existed for decades However, those super-big explosions are actually less tactically and strategically useful. More useful are smaller explosions that can be placed with more precision, and the tech needed to hit with precision has only improved over time. It is more likely that deployed weapons would have yields measured in kilotons. While bigger than Nagasaki, probably still closer to Nagasaki than Tsar Bomba