How is the punishment for polytheism (shirk), despite it being the gravest of sins, less severe than the punishment for adultery (zina)?

The punishment for major shirk (polytheism) is greater than the punishment for zina (fornication/adultery) in this world and the hereafter. The punishment for a major mushrik (polytheist) who was a Muslim is execution. He is not washed, nor is a funeral prayer offered for him, nor is he buried in Muslim graveyards. His punishment in the hereafter is eternal damnation in Hellfire. As for zina, its worldly punishment is lashing and exile for the unmarried, and stoning for the married. He is washed, a funeral prayer is offered for him, and he is buried in Muslim graveyards. His punishment in the hereafter is subject to Allah's will if he dies without sincere repentance; Allah may forgive him or punish him and then eventually release him from Hellfire due to his faith.