As such hardened targets are much more responsive to distance from ground zero than to the magnitude of the blast (i.e. warhead yield as measured in kilotons or megatons). 

So if one desires as much damage to hardened underground targets as possible, better to develop an arsenal of small, low yield, but highly accurate delivery systems than, say, the humongous 58 megaton monster the Soviets demonstrated when they showed their contempt for JFK, 8 months after he took office, by unilaterally ending the moratorium with a spectacular month long string of atmospheric test events, one per day, by way of flexing their nuclear muscle.