Crustal-stored magma reservoirs contain exsolved volatiles which accumulate in the reservoir roof, exerting a buoyancy force on the crust. This produces surface uplift and sudden loss of volatiles through eruption results in syn-eruptive subsidence. Here, we present three-dimensional, visco-elasto-plastic, numerical modeling results which quantify the ground deformation arising from the growth and release of a volatile reservoir. Deformation is independent of crustal thermal distribution and volatile reservoir shape, but is a function of volatile volume, density and depth and crustal rigidity. We present a scaling law for the volatiles’ contribution to syn-eruptive subsidence and show this contributes ~20% of the observed subsidence associated with the 2015 Calbuco eruption. Our results highlight the key role that volatile-driven buoyancy can have in volcano deformation, show a new link between syn-eruptive degassing and deflation, and highlight that shallow gas accumulation and release may have a major impact on ground deformation of volcanoes.