Why no single ship is the most efficient fleet

What is the most effective ship per pound in the 1920s?

There is no single answer: fighting strength favours a large fast capital ship, while global coverage requires numerous smaller ships and therefore a mixed fleet.

A fast capital ship with a heavy secondary battery can fight powerful enemies, but too few can be built to cover trade routes and overseas commitments. A mixture of battlecruisers, carriers, cruisers, and sloops balances combat concentration with presence and deterrence.