You sort of missed my point.
Ship size is determined by the physical sizes of the ship in metres. The orca is too long for a medium pad so requires a large pad and is designated as a large ship.
There is nothing to suggest that all large ships are similar in size.
That was just an example. IRL and in much of Elite, ships sizes do correlate with their capacities.
The orca occupies a very sensible spot in Elite. It's a passenger ship that costs half what the beluga does. Its meant to operate in places where you either cannot afford a beluga or you simply don't have the volume of passengers to warrant running a beluga but you want more efficiency than running a dolphin.
Most outposts don't have enough volume of passengers to fill an orca so you don't need the orca to be able to land on medium pads.
Again, that wasn't what I was going after, but I've never gone on passenger runs when I couldn't fill my Beluga.
Remember that these ships are designed by the ship building companies to fulfill a specific role in the Elite universe. The fact that we might want to rip all the cabins out of an orca and use it as some kind of gun ship simply isn't what its designed for.
Even Bentley and Maybach design their cars for specific roles, but they don't make the vehicle as large as a Semi.
Also remember that many ships have specific roles. There is NO requirement or game 'score' for you to grind to step up to a bigger ship. That's entirely your own choice that bigger is better. There are many many players that don't want an anaconda for instance because they prefer to fly a python which gives them a better all round ship in a medium class.
Again, not what I was aiming at - I was starting a discussion of where these specific role ships - even for their specified roles - fall down on the design side.
For example, I have fully stretched ASP-X and DBX ships that I use for exploration. But, I also have an Anaconda that is a combo courier/explorer. My Vulture and is definitely set up as a Fighter and my T9 Heavy is ONLY a big FedEx truck. But, why must there be two size 2 compartments in my Orca that could have been 4 size 1's to make the Stellar and Planetary scanners and the Docking Computer workable without wasting space?
Or, here's a thought - Given a ship's size and capabilities, why can't we decide what size compartments we install within the realm of what the ship can hold? Maybe I'd rather have the ability to install 2 size 1, 4 size 4, and 2 size 7? It still adds up to 32. Then, the available installable modules are limited to the vessel type in much the same way that we can only install fighter bays or military-specific components on certain ships.
As it stands, compartment assignment versus ship size? As @trollymctrollerson said - "There's not much rhyme or reason to some of these things."