The original names for the atom bombs that ended World War II were Fat Man and Thin Man and came from characters created by mystery writer Dashiell Hammett. Thin Man eventually became Little Boy but Fat Man — he was the Sidney Greenstreet character in Maltese Falcon — stayed.
I’ve read another explanation, which may be apocryphal-I cannot now recall the source. But it was that “Fat Man” and “Thin Man” referred to Mr. Churchill and Mr. Roosevelt. And when Roosevelt passed away, “Thin Man” was replaced with “Little Boy”, instead.