The difference is that it took 4000 tons of bombs to destroy Dresden, and 2 tons of bombs to destroy Hiroshima.

The other difference is that those 2 tons were the pinnacle of applied science at that time (heck in some way they are still pretty close to the _current_ pinnacle) and required the combined effort of hundreds of world-class scientists and engineers, unlike the 4000 tons of regular ordnance used in Dresden.