Harold Agnew with the Plutonium Core

Luiz Alvarez with the plutonium core
Luiz Alvarez and an MP with the plutonium core

The plutonium core (the box) in the Fat Man weighed 6.2 kg or about 14 lb, the pit is 9 cm (4 inches) across. And only about one fifth of it, a bit over 1 kg (2 pounds) undergoes a fission reaction. And only a gram (1/30th of an ounce) of that gets converted into explosive energy equal to 21,000 tons of TNT.