Why equal-caliber British and American torpedoes differed in weight

Why were British 21-inch torpedoes heavier and larger than American 21-inch torpedoes?

They carried different internal systems, warhead and propulsion choices, and features adopted from British testing.

Caliber fixes only the torpedo's outer diameter and does not standardize length, fuel, machinery, explosive charge, or reinforcement. British and American weapons could therefore share a nominal 21-inch size while carrying very different internal arrangements. British requirements for payload and propulsion added mass. Trials also encouraged construction features that made the weapon larger and heavier despite the common diameter.