This design system is effects-based. It's the effect that is important, not how you got there.
A TL 14 Grav APC is designed as a grav AFV with space for 14 troops and a VRF gauss gun. It has 50 armour on all faces, goes 400km/h, has a range of 3000 km, and is complexity 30. It has a hull of (say) 6, and structure of 7. Electrostatic armour provides another 28 points of armour versus plasma and fusion weapons, and 14 points vs. kinetic penetrators, while a laser-based anti-missile system can shoot down 5 targets a round. All stats with an effect in the game.
The hull material, weight, volume, power plant, and the like is subsumed within the design of the base chassis, which is why the system is so fast. The idea within the design system is that the engineers building the vehicles know what they are doing. All a player/referee really needs to do is decide what parts go into the mix.