Why the wartime Royal Navy doubled in manpower

Why was the Royal Navy so much larger in the Second World War than in the First?

The later war was more globally dispersed and required a larger Fleet Air Arm, shore establishment, and commando structure as well as more ships and convoy work.

The Royal Navy grew from roughly 400,000 personnel in the First World War to about 800,000 in the Second because the later conflict operated on a different scale and across a wider geography. The earlier naval war was global, but the main troop and supply effort remained more concentrated around Europe and the Middle East. The later war required sustained operations and convoys across far more theatres. Fleet Air Arm bases, Royal Marines and commandos, and the shore organisations needed to support a larger aviation fleet all added manpower beyond the crews visible aboard ships.