We analyze the impact of ideology on the size of government. In a simple model, the government sets redistribution and provision of public services according to the preferences of the median voter. Ideology is defined in terms of preferences for public services, and the impact of ideology on the size of government is shown to increase with mean income. This idea is tested using measures of ideology based on party manifestos. We show that the interaction of ideology and mean income has a major role in explaining the increase and divergence in government size observed across OECD countries.