Why a battleship hull is denser than a battlecruiser

Comparing warships by hull dimensions alone can obscure how differently their displacement is used. A battleship carries a much greater mass of armour than a battlecruiser or modern lightly protected combatant of similar size. That protection makes the battleship structurally and materially denser even before differences in weapons or machinery are counted. Length and beam alone do not reveal how much of a hull's displacement goes into protection and other combat systems.