Why Kirov received undersized reactors

Why did the Soviet Union choose undersized reactors for the Kirov class?

Designers thought they would suffice and began from submarine-reactor concepts before recognizing that a surface ship needed a different plant.

Designers initially believed the proposed machinery would be adequate and explored adapting submarine reactor concepts. Experience showed that a large surface ship requires a substantially different reactor arrangement and output, leaving the installed concept undersized for the role.