The cell contents are encapsulated within a compartment, the volume of which is a fundamental physical parameter that may affect intracompartmental reactions. However, there have been few studies to elucidate whether and how volume changes alone can affect the reaction kinetics. It is difficult to address these questions in vivo, because forced cell volume changes, e.g., by osmotic inflation/deflation, globally alters the internal state. Here, we prepared artificial cell-like compartments with different volumes but with identical constituents, which is not possible with living cells, and synthesized two tetrameric enzymes, β-glucuronidase (GUS) and β-galactosidase (GAL), by cell-free protein synthesis. Tetrameric GUS but not GAL was synthesized more quickly in smaller compartments. The difference between the two was dependent on the rate-limiting step and the reaction order. The observed acceleration mechanism would be applicable to living cells as multimeric protein synthesis in a microcompartment is ubiquitous in vivo.