Adventure Comics #270

Two Xelthunians secretly replace Ma and Pa Kent to perform a secret mission, not knowing that the Kents are Superboy's foster parents. After Ma Kent unwisely mentions Superboy's vulnerability to kryptonite, the Xelthunians power the transporter ray they plan to use to send Superboy to their planet with red kryptonite and it turns him into a fat Boy of Steel, a bullet-sized Boy of Steel, and a giant Boy of Steel. Once Superboy discovers their secret mission -- moving their sub-atomic world -- and helps them accomplish it, everyone goes home happy.