I see it differently...
Tiny Hull -> Speed Boat
Small Hull -> Corvette or PT Boat
Cargo -> Container Ship
Medium -> Cruiser/Destroyer
Huge -> Battleship/Carrier
Of course, this is debatable but certainly the cargo hulls theoretically carry more mass than any other ship and are able to handle whatever engines you choose if you are willing to trade the space for power.
As for the rest, I don't have a problem with putting the same type of engine on different ships as long as you take into consideration the power to mass ratio and the fact that the engines may be the same type but obviously they must be smaller. Think in terms of the jet engines on a fighter vs the engines on a 747.
They can't put enough technical nuance in the game for me but I will take what they give me