Why modern warships favor subdivision over heavy armor

Modern ships protect selected critical spaces, but rely heavily on compartmentation to contain fire and flooding after missile hits. A missile can carry both explosive and unspent fuel into the hull, making the spread of fire as important as the initial penetration. Designing armor to stop current missiles would encourage larger penetrators and impose a costly spiral similar to the historic contest between guns and armor. Lasers, railguns, or other future weapons could again reward larger battlecruiser-like platforms.