Hiroshima was just a single uranium bomb with (relatively) low yield that airburst, though, so hard to compare that to modern nuclear weapons. Modern total nuclear war exchange doctrine is pretty much: High-atmo detonation to create EMP, followed up by strikes on in order - Leadership locations, major military sites/ICBM sites, secondary military targets, large cities. You're talking about hundreds of multi-warhead missiles raining down literally all over the country, with each individual warhead containing probably at least 30-40x the power of Hiroshima...but when you factor in they pack like 8 into a single missile, you're looking at hundreds of times more powerful per launched weapon.