I'm treading lightly, based on how the last space-combat-realism thread turned out. Essentially, the weapon in space is kinetic energy -- not nuclear, not chemical, not beam (yet, though there's potential there obviously). All you need is mass and velocity -- if there's a chemical/nuclear role to be played, it's in mass acceleration. So if it was me, building an interstellar fleet to impose my empire, I'd be using asteroids as the offensive weaponry -- use drones to capture them and affix rockets, then drag them along with the fleet. When necessary, you give the command and they fly themselves to the target ship at ludicrous speed and obliterate it. Seems safer and more efficient than toting around giant bombs -- although you have to imagine you'd carry a few of those, too, in case you needed to do any atmospheric work.

So, is a 500 terraton bomb overpowered? I think so. I mean the dinosaurs went extinct from a giant rock. Still, blowing something like that up in the middle of an enemy formation is sorta like pushing that button from Last Starfighter, that kills everything in a galaxy radius or whatever. I mean you think frag grenades hurt here on earth, imagine if the projectiles never slowed down, if they just kept going forever until they hit something with the entire force of the explosion? That's what you get from a spacebomb.... it's a little chaotic for my tastes, and so primitive. I'll smash my enemies with rocks like a civilized gentleman, thank you very much.