Well, we have few "dedicated" ships, purely civilian, with only one real goal - providing cargo\fuel capacity.
In case of fuel capacity, fuel consumption is significant factor, not as significant as move speed, still.
Drum carry 300 fuel with max burn of 9 and 1 fuel per ly, this is 300eff
Phaeton carry 600 fuel, with same burn, and 2 fuel per ly, this is same 300eff
Prometheus carry 2500 fuel, with miserable burn of 6 and 10 fuel per ly, this is 250eff (2500\10)
IRL this works in absolute reverse, giant freightliner is much more effective compared to smaller vehicles, and carry a lot more cargo for it's own mass compared to smaller trucks and civilian cars.
Also it's not that slow compared to smaller cars and perfectly keep 110kmh on freeway, just like everyone else.
Basically, only phaeton worth anything as long as you not hit maximum number of ships, as it get 100 bonus from HRS and completely equal to drum in eff.
For cargo, main factor is supply consumption (fuel consumption also important, but in such case on Colossus worth it):
Buffalo 300 cargo and 3 supply 100 eff
Tarsus 300 cargo and 3 supply and 80 crew 100eff
Colossus 900 cargo and 6 supply and 200 crew 150eff
Atlas 2000 cargo and 10 supply and miserable burn level 50eff
As result, best ship colossus, as it provide 50% more cargo capacity per supply cost and additional 200 crew.
As result, getting largest and most costly ship in line, is not upgrade, not ever close, in case of atlas it's major significant jump backward.
Is this by design? or probably have some logic behind? probably this is reason of domain downfall?