Nukes aren't *that* powerful in many sci-fi universes. Consider an amount of antimatter equivalent to the mass of a normal bomb's nuclear load, for example, which would be a scientifically known factor. If I recall it correctly, the mass to energy conversion efficiency of a standard fission warhead should top off at about 0,4%, and a hydrogen fusion bomb at about 0,7%.
Other interesting but not really weaponiseable, at least in a conventional fashion, methods of harvesting energy out of matter are: dropping matter onto a neutron star with about a dozen percent efficiency, harvesting energy from a black hole's rotational speed, at about 29%, harvesting energy by dropping matter into a black hole's accretion disk and catching the resulting radiation, in the whereabouts of 10%-40%. Then the most efficient proccess is gathering a black hole's Hawking radiation, at just shy of 100% (due to astronomical timelines and the gigantic rise of emitted energy when the black hole nears the end of its life).
In comparison, matter-antimatter annihilation is theoretised to be capable of allowing for a conversion efficiency of about 50% minimum (due to vast amounts of heavy neutrinos resulting in the annihilation proccess escaping into space), theoretically up to also just shy of 100%, if you manage to specifically annihilate only electrons with positrons. I presume the latter to be achieveable in some sort of reactor but not in a bomb.
So comparing fission/fusion (taking the max fusion efficiency) bomb to the minimal antimatter reaction efficiency, there's over 70x more explosive power for the same amount of mass (of the payload itself, not counting the rest of the system) in an antimatter bomb.
A single antimatter Habringer would probably be able to crack if not snap a planet in half.