The Shrinking Superman is a comic book story appearing in Action Comics #245.
Superman brings Lois Lane to his Fortress of Solitude, so she can write a story about it for the Daily Planet. However, when she accidentally knocks over the Kryptonian Bottle City, she accidentally lets out Zak-Kul, a renegade scientist.
Zak-Kul is able to enlarge himself using Illium-349. He then gives himself plastic surgery to make himself identical to Superman, and takes a spare costume. He fools Lois, and when Superman returns, she can't tell the difference. Holding a Reducing Ray, she asks both Supermen to describe a criminal which the real Superman recently caught. When one of the Supermen fails to remember, she shrinks him. However, she has shrunk the wrong Man of Steel!
Zak-Kul actually continues Superman's heroism in Metropolis, rather than trying to commit crimes using his super-powers. He also marries Lois Lane, which the real Superman was never willing to do. However, when she begins trying to learn his secret identity, the imposter arranges to kill her. The real Superman, meanwhile, has found a way to escape from Kandor and enlarge himself, and arrives in time to save Lois.