The durability of solid oxide cells is critically limited by the overall thermochemical expansion of perovskite electrodes, exacerbated by the formation of oxygen vacancies with increasing temperature. This creates a fundamental trade-off, where oxygen vacancies enhance electrochemical functionality at the cost of thermomechanical stability. We independently study thermal and chemical expansion of fifteen perovskite oxides with complex A-site cation mixtures using synchrotron X-ray diffraction, dilatometry, thermogravimetry, and density functional theory. We demonstrate that the Goldschmidt tolerance factor governs both thermal and chemical expansion through octahedral tilting dynamics, which offset bond-stretching vibrations and compensate for vacancy-induced strain. We identify that high-entropy A-site mixing also suppresses chemical expansion via "strain containment", where larger cobalt sites with increased bond covalency locally absorb vacancy-induced strain. This lowers the chemical expansion coefficient by up to 29% relative to the benchmark electrode. Together, these correlations and mechanistic insights map structural and compositional descriptors onto expansion behaviors, providing targeted design principles to tune thermochemical expansion without sacrificing functional defects.