Two of the most widely observed co-eruptive volcanic phenomena - ground deformation and volcanic outgassing - are fundamentally linked via the mechanism of magma degassing and the development of compressibility, which controls how the volume of magma changes in response to a change in pressure. Here we use thermodynamic models (constrained by petrological data) to reconstruct volatile exsolution and the consequent changes in magma properties. Co-eruptive SO₂ degassing fluxes may be predicted from the mole fraction of exsolved SO₂ that develops in magma whilst stored prior to eruption and during decompression. Co-eruptive surface deformation may be predicted given estimates of erupted volume and the ratio between chamber compressibility and magma compressibility. We conduct sensitivity tests to assess how varying magma volatile content, crustal properties, and chamber geometry may affect co-eruptive deformation and degassing. We find that magmatic H₂O content has the most impact on both SO₂ flux and volume change (normalised for erupted volumes). Our findings have general implications for typical arc and ocean island volcanic systems. The higher magmatic water content of arc basalts leads to a high pre-eruptive exsolved volatile content, making the magma more compressible than ocean island eruptions. Syn-eruptive gas fluxes are overall higher for arc eruptions, although SO₂ fluxes are similar for both settings (SO₂ flux for ocean island basalt eruptions is dominated by decompressional degassing). Our models are consistent with observation: deformation has been detected at 48% of ocean island eruptions (16/33) during the satellite era (2005-2020), but only 11% of arc basalt eruptions (7/61).