Little Boy was ~15 kilotons - quite a bit smaller than Trinity was at 20kt or Fat Man at 21kt. So Hiroshima wasn't even that big for a nuclear weapon of the time. Today we measure in megatons. Small nukes today will probably larger than Hiroshima size blast. your point stand though... small-scale (regional) nuclear war will be recovered very quickly, mostly because global efforts at recovery. if the global recovery systems are broken (as they are today...) then recovery won't go as quickly.