Why did Cold War weapon proposals become so extreme?

Was Cold War weapons planning effectively insane when proposals included missiles with enormous nuclear warheads?

Often, yes: fear of what the enemy might possess allowed advocates to justify and pursue exceptionally extreme systems.

The question cites a British surface-to-surface missile proposal associated with a roughly 7,000-pound nuclear warhead. The answer agrees that Cold War planning could become extreme. The explanation is institutional: if advocates could justify a weapon against a presumed enemy capability, they could often make a case to build it. Persistent uncertainty about what the opponent possessed widened the range of systems that planners could portray as necessary.