There is a great interview with Robert McNamara in the documentary "Fog of War". He was a bomb damage analyst during WW2. He said that by that point in the war we were killling 100k Japanese civilians a day by firebombing. On the day of Hiroshima, we killed 45k but we did it with a single bomb (3 planes in the air), instead of 100k with 1000 planes as was the custom. If we wanted maximum deaths we could have sent them all and killed 145k that day. We did not. We were going for psychological impact, NOT maximum deaths.