" The hydrogen bombs we have now are already so large that no legitimate military target could ever exist which called for one."

Which is why all of the large ones have been decomissioned.

Ironically, eliminating "bigger" bombs in favor of smaller ones is being cited as a danger in and of itself. Because the threshold to using nuclear weapons currently is so high (even in the 170-300kt range as they are now), pursing accurate weapons in the tens of kilotons range is seen as destabilizing because it's seen as being more acceptable to use against say, a remote, hardened target.

Could be useful for blowing up an incoming asteroid. We're currently stuck in a dilemma where if the rock is small enough for a missile to actually wipe it out, it won't be dangerous enough to merit nuking in the first place. If we had a reliable and lightweight bomb that could take out a rock the size of Ceres, we would have a much better chance of defending our planet. Of course we would still have to refrain from annihilating ourselves.

Warhead assemblies for the B53 (9mt) are thought to be retained for this purpose: https://en.wikipedia.org/wiki/B53_nuclear_bomb#Role