This paper focuses on the role of development in informality through higher wages and expanded production possibilities. First, it uses informal, plant-level survey data across countries to document that on average, richer countries have smaller informal, unregistered plants in terms of employment. This negative relationship holds even after controlling for plant-level characteristics. Then, a dynamic general equilibrium model with incomplete tax enforcement is developed such that formal and informal plants coexist in equilibrium. The model allows for two groups of agents operating in the informal sector: those with lower abilities than workers, and those with abilities falling between workers and formal managers. In the model, when plants become more productive, some agents operating informally choose to be workers and some of them transition into formality due to higher wages and better production possibilities, which decreases the mean size of informal plants. The quantitative results indicate that around 30 percent of the increase in aggregate output due to higher productivity is associated with a roughly one-quarter decline in the mean size of informal plants.