[quote=So Boerd]
No, one kilogram blows up with 53 megatons. It annihilates not only itself, but a kilogram of matter.
[/quote]

How do you get 53 megatons out of that? 21.5 x 2 = 43. 

Though that's not how it works anyway. The kilogram of antimatter collides with the kilogram of matter and they both annihilate each other to produce an explosion with 21.5 megatons of force. The explosion (and calculation of force) happens after they're combined, not before.