An important result in interest group theory and political economy is that small groups are more influential than their size would lead us to expect. In this study, we document that the opposite holds for political mobilization. Citizens are more likely to participate in elections if they belong to large groups. We present evidence that both the absolute size of groups and their population share influence individual participation decisions. The link between group size and political mobilization is in part due to the structure of media markets. Candidates find it easier to direct campaign messages at larger groups because many existing media outlets cater to them.