From published data we found characteristic relations between the amount V of erupted lava and the duration d of eruption for Mount Etna, Kilauea, and Piton de la Fournaise. The relation is similar between Mount Etna and Kilauea, where the increase of V with increasing d is slow, showing a trend of a lower flow rate for a larger eruption. For Piton de la Fournaise, however, the trend is distinctly different, showing a higher flow rate for a larger eruption. We constructed quantitative models of a magma system with reservoirs at various levels and tested hypotheses about the existence of large reservoirs under these volcanoes using the observed V-d relations. We found that the observed V-d relation is consistent with the presence of a large reservoir at a shallow depth under Kilauea, and with the presence of a large reservoir near the bottom of the volcanic edifice under Piton de la Fournaise. The above models for Kilauea and Piton de la Fournaise are characterized by a simple path from the mantle reservoir which leads to the shallowest reservoir connected to a hierarchy of channels with varying resistance to eruption sites. We obtained less satisfactory agreement between the observed V-d relation and the predicted using our model for Mount Etna. Time histories of pressures in the reservoirs at various levels obtained by the modeling explain why inflation-deflation cycles observed at Kilauea have not been reported for Piton de la Fournaise. The absence of volcano-tectonic earthquakes with magnitude greater than ~2.5 under Piton de la Fournaise is attributed to the simplicity of the magma path from the mantle to the shallowest reservoir and the underdeveloped rift zone, which result in a stress concentration localized only beneath the summit area. ¿ 2001 American Geophysical Union