The Soviets had (and the Russians still maintain) the one anti-ballistic missile system allowed by treaty. It's emplaced all around Moscow. In addition, Soviet leadership had constructed a series of deep bunkers and transport systems under Moscow -- there is/was another subway system underneath the civilian Moscow system for Kremlin leadership to use.

So should a missile (of several targeted against Moscow) make it through the ABM system, it would still have to destroy a very hard target. And so you need a big bomb to ensure destruction.