Sizing an amphibious ship from its mission

Why would a purpose-built helicopter assault ship be smaller than a converted carrier used in a similar broad role?

Displacement follows the required troops, landing craft, aircraft, flight spots, and operating concept, not the tonnage of a different ship already in service.

The design starts with the mission: how many troops must embark, what landing craft are needed, how many aircraft must be carried, and how many can operate at once. Those requirements determine the hull rather than a desire to match a converted carrier's size. Existing dockyard infrastructure can make a given displacement easier to support, but it does not by itself define the correct ship.