“…Field studies of plutons (C. F. Miller et al, ; R. B. Miller & Paterson, ; Paterson & Miller, ; Wiebe, ; Wiebe & Collins, ) and eruptive deposits (Chamberlain et al, ; Charlier et al, ; Matthews et al, ), analogue experiments (Snyder & Tait, ), and numerical models (Annen, ; Jellinek & DePaolo, ; Karlstrom et al, ) have illuminated the physical processes by which magma chambers are assembled, typically by the episodic injection of magma via dikes ascending from deeper reservoirs. However, we do not yet understand what determines the proportion of magma that ultimately remains in the crust relative to the amount erupted (Black & Manga, ; White et al, ) and how this relates to the long‐term growth of eruptible portions of the reservoir, referred to here as magma chambers. Thermal and mechanical models (Annen, ; Degruyter & Huber, ; Jellinek & DePaolo, ) suggest that for a given rate of magma supply, smaller chambers are more likely to erupt and freeze, while larger chambers favor magma storage, which begs the question: How do magma chambers grow to the volumes required to sustain the largest eruptions on Earth?…”