Why barbette width does not dictate turret size

Why can a twin 15-inch turret look small relative to the width of its barbette?

The barbette's size reflects what must fit and work below the turret; a wider gunhouse would change the gun angles and other operating constraints.

Barbette width depends on the equipment and working arrangements below the visible turret. A relatively small gunhouse therefore need not indicate a simple attempt to save turret weight. A wider mounting, potentially even a triple turret, might fit the barbette, but it would alter the gun angles and complicate the arrangement. The design aims to obtain the best use from the chosen turret and the space beneath it.