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).
Mainland US flights were unable to reach Japan, even from Hawaii or Alaska. Large-scale effort required control of a large portion of the Pacific and both launch and recovery capabilities.
A significant number of aircraft for the Doolittle raids (early in the war) were ditched before they could reach their designated LZs in China. The aircraft, B25B Mitchell medium bombers, could launch from carriers (easier to arrange for than island occupation), but lacked either the range or raw capability to land on the Hornet. The raids were effective as a shock and demonstration of US reach and capabilities, but had minimal direct military effect.