This study explains why two contemporary European frigate programmes—the British Type 31 and the German F126—developed into larger and more complex warships despite explicit ambitions for simplicity and cost control. Using theory-testing process tracing, the study evaluates competing Path Dependence and Rational Choice explanations across the procurement processes. The analysis shows that smaller platforms were technically feasible but rendered institutionally unrealistic by entrenched standards, doctrinal assumptions, regulatory requirements, and industrial continuities that constrained decision-making early in the process. While rational arguments were present, they primarily served to justify outcomes shaped by these structures. The study concludes that Path Dependence provides the most convincing explanation and highlights the difficulty of achieving transformative change in defence procurement without early structural reform.