– Why did the weaker of the two bombs, ‘Little Boy’ reportedly caused more deaths than the stronger ‘Fat Man’?
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The death and damage caused by a bomb is not only determined by the absolute strength of the bomb. The height that the bomb is released, the local geography and construction, the density of inhabitation, and the response afterwards all have a large effect.
For example, Hiroshima had an estimated 350,000 residents before the bombing, to Nagasaki’s 260,000. Another major factor is that Hiroshima spread across a wide, flat plain, while the spot that Fat Man detonated was in a small valley. This means that the blast at Hiroshima was better able to spread out and do damage, whereas much of Nagasaki was partially shielded by the land.
There were more people under little boy when it was dropped
In other words: A 50 cal shot into a rock in the desert will kill less people than a 22 shot into somebody’s skull.
Two main reasons
First, Geography, Hiroshima was a flat open space, the blast was on target and spread out well. Nagasaki is located in a valley that the blast wasn’t’ as impactful
Second, aiming… they pilots probably missed their target on Nagasaki because of bad weather and feeling forced to drop the bomb. They claimed everything was fine with weather (historians aren’t so confident) … but they probably missed their target and hit a poorer location, so the bomb wasn’t as effective due to the geography of Nagasaki being in a valley
First, the power between the bombs was not that significantly different. The damage caused by nuclear weapons does not scale linearly; 20kt and 15kt are basically similar (the 5 psi blast damage area for Little Boy was 3.4 mi/sq, for Fat Man it was 3.6 mi/sq, for example). If the yields are essentially the same order of magnitude, the damage will be basically the same.
Second, Hiroshima was sort of an ideal target for the atomic bombs of World War II. It was basically a bowl-shaped city that was the same size as the damage radius of the bomb. The bomb was placed correctly in the center of the city, and so was able to deliver its damage very effectively.
By comparison, Nagasaki was already an inherently poor target (it is a V-shaped city with two parts by a mountain) and the bomb wasn’t placed in the right place anyway (it was placed deep inside one of the ends of the V, so the mountains shielded the other half of the city; it was meant to be placed more at the bottom of the V). The part of Nagasaki that got hit got hit very hard, but the other part of it — which included its main city center — did not. [This graphic](https://blog.nuclearsecrecy.com/wp-content/uploads/2017/01/Hiroshima-Nagasaki-map-scaled.png) gives a sense of how different the areas affected here; note that this is resized to a constant scale (the originals it was based on were not, which made them look the same). Nagasaki, as an aside, was the lowest-priority target for the atomic bombs for these kinds of reasons. The actual target for the second bomb was meant to be Kokura, which was geographically much more like Hiroshima, but [clouds or smoke obscured the target](https://blog.nuclearsecrecy.com/2014/08/22/luck-kokura/), so the bomber went to the backup target, Nagasaki.
Third, they had different populations to begin with. Hiroshima was estimated around 255,000 people living in it at the time of the bombing; Nagasaki had 195,000 (the exact estimates vary, but the general proportions don’t). [More details on the (tricky to make) casualty estimates here](https://thebulletin.org/2020/08/counting-the-dead-at-hiroshima-and-nagasaki/). This is incidentally also the explanation of why the Tokyo firebombing killed more people than the atomic bombings, even though [atomic bombs are much more deadly in terms of how many people they kill in the affected area](https://blog.nuclearsecrecy.com/2014/09/22/tokyo-hiroshima/): Tokyo had a population of millions, and so even a small percentage of that killed is a very large number. Around 7% of the people in the targeted areas of Tokyo were killed by the firebombings, and 50% of the people in Hiroshima were killed by the atomic bomb, but because the base populations were so different they come out to be about the same number total.
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