Missile Equal to 20,000 tons of TNT"
"Little Boy" dropped on Hiroshima was a smaller "gun-type" assembly, which yielded "only" about 15,000 tons TNT equivalent.
"Fat Man" heading towards Nagasaki is the more advanced implosion type with plutonium core.
It will, indeed, yield around 21,000 tons equivalent.
A scale model of "Little Boy"
Contrast to "Fat Man"
