It seems to be about right--the predicted damage to Hiroshima from Little Boy matches up pretty well with observed damage (see http://www.atomicarchive.com/Maps/HiroshimaMap.shtml)

I think it's just a very different geographic scale of the cities involved. We learn that Hiroshima was destroyed by a single bomb; we don't often hear that Hiroshima is tiny in comparison to e.g. LA (which would require something on the scale of Tsar Bomba, or multiple strikes, to destroy it to a similar degree).