It’s so odd to see that an object roughly the same size as an SUV can destroy an entire city and leave radioactive damage for lifetimes afterwards. The Fat Man and Trinity were both implosion based plutonium devices, but they were very different weapons. Trinity’s “the gadget” was a massive unemployable bomb that would test the theories that Fat Man was based on. We didn’t know Fat Boy would detonate or not.
Little Boy was just a simple gun type bomb, but numerous countries would use gun type bombs into the 70s. The reason Little Boy wasn’t tested was because we couldn’t produce enough HEU (Highly Enriched Uranium). The massive Y-12 complex and K-25 complex had only produced enough HEU for one bomb at the time. 20 years later they had a bomb that would produce a similar yield but could (barely) be carried in a backpack. What’s really wild about this bomb is that they only built it 30 years after the stopped filling artillery shells with black powder.
The Fat Man was substantially safer than Little Boy in that regard, since its pit had to be compressed by the high-explosive lenses before the neutron flux from its initiator would be sufficient to start a chain reaction. In the middle of Fat Man there was a core of Plutonium with several other layers building up to the outer layers of explosives. The explosives, when set off all at the same time would compress the inner layers of radioactive material to an extreme level starting a chain reaction of fission. It is estimated that only 20% of the 13.6 lbs (6.1 kg) of Plutonium went critical. 2.72 lbs (1.2 kg) of Plutonium wiped out an entire city
The Fat Man was 21 Kilotons. 1 Kiloton = 1,000 tons of TNT. 1 Megaton = 1,000,000 tons of TNT. The total energy of all explosives used in World War Two (including the Hiroshima and Nagasaki bombs) is estimated to have been 3 megatons of TNT. The biggest one ever Tsar Bomba yielded over fifty megatons. Actually it was a 100 megaton design. The test version wasn’t full yield.