Why is leadership succession highly institutionalized in some polities but not in others? We propose that the size of the polity constitutes a key explanatory factor. Specifically, we argue that an institutionalized process of succession is more likely to be adopted in larger polities because there are more elite actors vying for power, making it difficult for a single actor to consolidate power, hold it indefinitely, and pass it on to his heirs. To test this argument, we construct a global index centering on observable features of leadership succession. The index, drawing on data from the Archigos project, covers most sovereign countries from the mid-nineteenth century to the present. A battery of cross-national tests shows a positive and robust association between polity size and the institutionalization of leadership succession.