I think superpowers accumulate atomic bombs as an element of dissuasion, not really to use them. Nowadays the scenario of a nuclear war seems unlikely because it would take the world to the apocalypse. Even when anti-missile shields have been developed, it would only take 1% to fail for a global disaster to occur. Current atomic bombs are much more powerful than those that were dropped on Hiroshima and Nagasaki. The one of Hiroshima was of 15 kilotones, the one of Nagasaki of 21 kilotones. Today there is Castle Bravo of 15,000 kilotones and the Tsar Bomb of 50000 kilotons. In this graph you can see the comparison based on the nuclear fungus that these bombs produce.