I did some rough estimates of ship masses just to see how dense some Btech warships are. I also put an illustration showing an Aegis at canon size compared to it at tank and naval ship densities. I'm thinking they could either be built like Tanks since they don't need to be buoyant or like naval ships if they need to be roomy enough for light but bulky equipment and crew quarters.
Even at only 243 meters in length the Cruiser has about the same density as most Btech ships. It looks tiny but that compact frame holds a lot of volume.
Cruiser
Weight: 500,000 tons
Volume (est.): 4,294,164 m^3
Density: 116 kg/m^3
Aegis
Weight: 750,000 tons
Volume (est.): 9,639,000 m^3
Density: 78 kg/m^3
Nimitz
Weight: 97,000 tons
Volume (est.): 415,000 m^3
Density: 233 kg/m^3
Abrams
Weight: 62 tons
Volume (est.): 51 m^3
Density: 1,208 kg/m^3