We use 2006–2009 ALOS Interferometric Synthetic Aperture Radar data over the entire west Sunda arc, Indonesia, home of 13% of the world's active volcanoes, to derive arc-wide time-dependent ground deformation data. We present unambiguous evidence of inflation at six volcanoes, three of which erupted after the observation period. We show that these volcanoes have shallow magma reservoirs at ∼1–3 km depth below the average regional elevation. A global comparison of reservoir depths at arc volcanoes suggests that volcanoes in extensional and strike-slip settings (west Sunda) can develop shallow reservoirs whereas volcanoes in compressional settings may lack them. Thus, magma ascend through the upper crust could be influenced by intra-arc tectonic settings.