Feeder dikes bring magma to the surface; non-feeder dikes become arrested and never reach the surface. The differences, if any, between these dike types remain largely unexplored because in the field it is normally unknown if a particular dike is a feeder or non-feeder. Here we present measurements of feeder and non-feeder dikes exposed from depths of >200 m to the surface in the walls of the A.D. 2000 caldera collapse of the Miyakejima Volcano, Japan. A typical feeder thickness reaches a maximum of 2–4 m at the surface, decreases rapidly to ∼1 m at a depth of 20–40 m, and then remains constant to the bottom of the exposure. By contrast, a typical non-feeder thickness reaches a maximum of 1.5–2 m at 15–45 m below the tip, and then decreases slowly with depth to 0.5–1 m at the bottom of the exposure. We propose that free-surface effects and magmatic overpressure (driving pressure) changes during the eruption cause the overall shape of a feeder to differ from that of a non-feeder.