>2 devices used on Japan. the reason "they" were so big and heavy, was because of the cannons inside them.
Just a minor technical note here, the Hiroshima bomb was a Uranium 235 "gun" type weapon, where one subcritical mass was indeed shot into another, using a gun-type setup. However, the Nagasaki bomb was an implosion device. The subcritical plutonium core was made supercritical by compressing it with precisely-shaped explosive charges that were detonated simultaneously.