Fascinating thread; Have not done anything in the Traveler universe in twenty years (or more); Recall discussing the disconect with Marc, is commerce in the Universe scads (Dozens? Hundreds? Thousands?) of little type M subsidized merchants, because even a single (50,000t?) bulk carrier can carry as much as an add on. And this was before we had fully articulated the "Container Revolution." What are the dimensions of a standard shipping container in the Traveller universe (T5?).
Both of those are long-fought-over questions. I don't know about the container size, but I do know that commerce in the Traveller Universe has both large freighters (100,000 tons) and small traders (200 tons) and a number of ships in between.
If I had to guess, I'd say that the Big Freighters are split between barge-carriers and container-shippers. A barge or lighter has a bit of overhead for grappling that might be bad for bulk cargo, but not a problem for high value cargo.