Radar horizon helps separate destroyers from frigates

Are warship sizes oscillating around a stable point rather than continuing to grow?

No fixed size governs growth; mission does, and air-defence destroyers tend to need larger, higher structures than frigates built around underwater sensors.

Mission matters more than a universal displacement curve. A frigate centered on sonar and undersea sensors can arrange much of its defining equipment low in the hull. An air-defence destroyer needs radar high above the water to extend its horizon. Supporting that elevated weight, power, and structure is one reason destroyers tend to become larger than frigates even when both classes grow over time.