Stirling engines are a perfect illustration of what I call Odum's Paradox:

  The closer a heat engine/pump gets to maximum theoretical efficiency, the lower its power density.

This is a simple consequence of thermodynamics. By approaching the Carnot efficiency you're asymptotically approaching an adiabatic cycle, so there's less and less entropy gain to drive the system forward. At the Carnot efficiency the cycle becomes completely time-reversible and all power stops, so as you get closer and closer the power density drops toward zero.

Odum observed[0] that (for certain linear assumptions) the maximum power density is achieved at half the theoretical Carnot efficiency. He noted that both real engines and biological systems tend to cluster around this optimum.

What's interesting, but not terribly surprising, is that this implies a fundamental and unavoidable tradeoff between machinery cost and operating efficiency.

[0] https://en.wikipedia.org/wiki/Maximum_power_principle