Nitrogenated heterocycles comprise the cores of a number of synthetically useful compounds, including pharmaceuticals, bioactive natural products, agrochemicals, and other drug-like molecules. The widespread interest in methods to increase the fraction of sp³ carbon atoms (Fsp³) of drug-like scaffolds in a stereocontrolled manner, while enabling explorations of unusual amine chemical space, inspired our efforts to tune the reactivity of aziridinium ylides. A sequential nitrene–carbene transfer of simple allenes leads to divergent product outcomes depending on the nature of the carbene precursor, furnishing products of different ring sizes. In addition, the catalyst control over the ring size via proposed hydrogen-bonding interactions between the catalyst and substrate was explored. Computational studies were employed to gain insight into the major features of substrates and catalysts that influence the tunable reactivity of aziridinium ylide intermediates formed in this chemistry.