Why navies moved from 18-inch to 21-inch torpedoes

Why did twentieth-century navies shift from 18-inch to 21-inch torpedoes?

The larger diameter provided more internal volume for explosive charge, propulsion, range, and machinery intended to make the weapon quieter.

A torpedo has to carry its warhead, engine, fuel or energy store, guidance, and supporting machinery inside one pressure body. Increasing diameter gives designers more volume to distribute among those functions. The main gains are explosive effect, power, range, and quieting. Calibre growth can be attractive even before changes in guidance or tactics are considered.