Hmm, that does explain why a mere handful of ships is considered a 'battlefleet'. (any space 4x game would scoff at a 'fleet' of a mere 20 ships)

They need to make lots of freighters. And then their production base isn't good at all and coordination is bad. Shortage of vital parts or materials holds up construction, as well as corruption, crime and just plain inefficiency.

And then the frieghter captains are formed by random current crew of a freighter who have earned enough to collectively make a deposit for a loan to buy a new freighter and strike out on their own. So there aren't shipping corporations per se, just a large number of freighter collectives, maybe headed by a Rogue Trader or noble (who hands down shipping contracts to trusted captains who then sub-contract out to other less well known freighter collectives who then divide it amongst themselves; feudal style and incredibly inefficient).

Which brings me to an idea that the Rogue Trader really ought to buy frieghters. Me thinks that would be rather easier to get and would move much much more mass than a converted Lunar or other warship. With the hyperspace drive, he doesn't need to worry about them being ambushed, and besides, he does have two converted warships now.