My in-universe explanation is that Bulwark was created as a huge armored transport ship, with several hangars an a lot of cargo space.
After the war started,the Techno Union changed it to be a cruiser like the Providence or the Recusant.
And finally the Rebels used the Mk3 as a moving space station.

The MkI really reminds me of a horseshoe crab... it's wonderful (and so are the others!)

While the multinational corporation can claim the Dutch and British East India Companies as ancestors, the modern corporate nation-state was forged during the First World War. It's not some inheritance from time immemorial; every piece of it is contested and its future is unwritten