Little Boy is around 20,000 smaller than the largest nukes the US has ever made.
Kinetic energy equation is 1/2*m*v^2
1 Kiloton of dynamite is 4.184x10^12 joules of energy
A little bit of algerbra determines that a 3 ton object going over 56,000 meters per second would have more power than the "Little Boy" bomb. To make it 10,000 times larger just increase velocity by 100 fold (since velocity is squared). That means it would have to be going 5,600,00 meters/second, or approximately 1/50th the speed of light. I don't know if objects fly through space at that speed though... I just know how to work the math.