https://en.wikipedia.org/wiki/Bombing_of_Tokyo

Dresden required nearly 1,300 bombers.

https://en.wikipedia.org/wiki/Bombing_of_Dresden_in_World_Wa...

The ultimately produced bombs were deliverable via B-29, as that aircraft was available. The carrier-launched Mitchell B25B was also available and had a payload capacity of 3,000 lb., though that is 1/3 the weight of Little Boy (the Hiroshima bomb). Whether or not a smaller package, or alternative delivery vehicle, could have been provided, I'm not sure. Though the challenge might have been surmountable.

https://en.wikipedia.org/wiki/North_American_B-25_Mitchell

https://en.m.wikipedia.org/wiki/Little_Boy

Casualties per mission over the European theatre were tremendous. I don't recall the specific per-mission losses, but overall crew survival over a 25 mission tour was on the order of 50%. Japan's defences were less effective, but again, looking at use of atomic weapons generally, the statistics are relevant, and during the planning of the Manhattan Project, use of the bomb against Germany was considered.

http://www.ww2f.com/threads/bomber-crew-survival-rates-durin...

There's also the possibility that smaller flights might escape detection (smaller radar target).