Why were the Walkers given lighter sentences when they commited the same offence?
John Walker was sentenced to two life terms plus ten years. His brother was given a life term. Jerry Whitworth received a total sentence of 365 years for his part in the Walker ring. Interesting idea of "lighter" you have.
Why werent they all executed is a better question. Presumably the Walkers briefed the FBI on what secrets they sold to the Soviets in return for leniencey. Dont know if Pollard ever did so. He certainly never showed any sign of remorse other than over getting caught.
Im troubled by your comment "they committed the same offence". From what I recall about the case, Pollard's betrayal resulted in a greater loss to the US than anything any other spy had ever been able to accomplish. That may also be reason why he got a longer term. Then it could be that Bush 41 wanted to make sure that Israel got the message that we werent going to turn a blind eye to them selling our top secrets to the Soviets.