I’d love for some non-economic reason for a small ship/CT77 universe. But at the scale of the imperium I’m not sure it really makes sense.
Hum, if one looks at the bulk of terrestrial shipping, a lot of commercial traffic actually is in relatively small vessels when compared to the few big transports. Note the very largest container ship and tankers have relatively few ports that they can operate out of. I.e. the Maersk Triple E has a half dozen ports in Asia, Rotterdam and couple other ports in Europe and none in North America. Also consider the Triple E there are 20 in the class in a field of 25,000 commercial vessels.
As for the justification of Book 2 based universe, How big do your ships need to be? Consider the average world population is Five...
Note consider the port cities like San Francisco and London at the turn of the 20th century. Hundreds of ships in each with significant numbers entering an leaving port every day.
Mostly the question is based on the material offered in the core book. And how to structure a game around what is presented.