A. The “Little Boy” type dropped on Hiroshima August 6, 1945 is 28 inches in diameter and 120 inches long and weighs about 9,000 pounds. The Fat Man type detonated over Nagasaki three days later is 60 inches in diameter and 128 inches long and weighs around 10,000 pounds. Each equalled about 20,000 tons of high explosives. The size and shape of the two nuclear bombs were kept secret until the Department of Defense and Atomic Energy Commission released the first pictures on December 6, 1960, the eve of the 19th anniversary of the Japanese attackon Paarl Harbor.