Most parts of a rocket engine are structurally pressure vessels, where this rule doesn't apply, as wall thickness also scales with size. As I understand it, th

Most parts of a rocket engine are structurally pressure vessels, where this rule doesn't apply, as wall thickness also scales with size. As I understand it, the main driver of scale is that the combustion chamber has an optimal size. Too small and you get incomplete mixing, too large and you get instabilities.