Why component-only treaty limits are hard to enforce

What if cruiser treaties limited armour and firepower but not fuel and machinery?

Such rules would be very hard to police because builders could hide protection and displacement in supposedly structural material.

A component-only rule would cap guns, armour, and fire control while leaving machinery, fuel, and structure outside the limit. A builder could exploit the definition by making structural steel unusually thick and insisting that it was hull material rather than armour. A nominally compliant cruiser could therefore reach roughly 18,000 tons while assigning much of its protection to an unregulated category. Treaty effectiveness depends on auditable definitions as much as on the numerical ceiling itself.