[kwekly]

Pure speculation, but I think heat dissipation is probably the more important bottleneck of spaceship design -- no convective cooling, so you potentially want more surface area to increase radiation.

I think aerodynamics (both lift to weight and streamlining) are the reason for the compact design of aircraft and submarines (also perhaps reducing the profile for military vehicles).

Weight maybe an issue for maneuvering in combat, but there's tons of structural material literally floating around in space, so I'd wager that base material cost would not a limiting constraint.