You are describing hangar space, not the volume of the craft itself.
With your method a Scout ship would be 37.5 m × 24 m × 9 m (three decks) = 8100 m³ ≈ 580 Dtons.
Displacement is literally the amount of liquid it would displace if dipped into it, i.e. the volume of the craft itself.
A generic hangar, i.e. not form-fitting, able to dock most spacecraft is 300% of the craft volume (HG2017, p60), much larger than the craft itself, despite not really assuming massively protruding wings of the craft.
An aircraft is a vehicle, look to the Vehicle Handbook, that separate payload mass ("spaces") and packing volume ("shipping tons"). An F-35 would perhaps be something like a 20 Space Light Jet, requiring a 20 Dt packing crate?