I seriously doubt if anyone, least of all NK, would be throwing 20Mt bombs at cities - the trickiest bit with |H-bombs isn't so much building big bombs but building small enough weapons that are easy to deliver by a missile.
The Soviets did have very large missiles (SS-18s) with an option for single very large weapons (20-25Mt) but those were for deep bunkers rather than cities.
> the trickiest bit with |H-bombs isn't so much building big bombs but building small enough weapons that are easy to deliver by a missile.
It's the opposite. Medium warheads (100~500kT) are more efficient[0] and more flexible, the original systems were huge because they were bomber-delivered, there was therefore an assumption that few bombs would reach the target and you wanted every one of them to pack as much punch as possible (no point in putting lots of small ones in a bomber as they'd all be dropped onto the same place anyway, when your explosion has an effect diameter in kilometres multiple bombs are just going to interfere with one another).
The advent of ICBM and later MIRV allowed splitting reducing the size of individual warheads as the delivery mechanism is both cheaper and more effective, thus you can have more of them and a larger fraction will reach the objective.
[0] ground effects grow sub-linearly with yield, if you increase yield from 20kT to 1MT (50x) the blast range grows by ~4x (changing burst height to optimise ground effects) and the affected area grows by ~15x
What I meant was that once they worked out staging building arbitrarily large weapons was actually fairly easy - AFAIR the team building the Tsar Bomba were criticized by their colleagues because what they were doing wasn't actually useful technically and that the real challenge was building smaller easier to deliver designs - the US W88 currently being the most advanced design that has been described in public.
I seriously doubt if anyone, least of all NK, would be throwing 20Mt bombs at cities - the trickiest bit with |H-bombs isn't so much building big bombs but building small enough weapons that are easy to deliver by a missile.
The Soviets did have very large missiles (SS-18s) with an option for single very large weapons (20-25Mt) but those were for deep bunkers rather than cities.