This pretty much jibes w/ what I've always imagined.
The largest civilian freighters would have no more than 500dton capacity.
I've never considered the 'just-passing-thru' ones into my calculations though, as I never considered them to have much local impact.
Unless, there's some sort of imperative, they just refuel & jump again.
"No point in hanging around, time is money & I'm on a deadline!"

-------------------------------------------- -----------------------------------------

 On 24 August 2014 17:54, Tim <tim@little-possums.net>
 wrote:
 I rolled a random third-world country, in this case getting
 Myanmar (Burma).  As of 2013, imports were 20% of its $60 billion
 GDP and exports 22%.  In Traveller terms this would be a pop-7
 world.  If we equate 2013 US dollars to 0.1 imperial credits and assume an
 average 20 kCr per dton cargo value, then a corresponding world
 would import and export about 1200 dtons per week.  Assuming a few days
 turnaround at each end, at any given time a good fraction of that
 combined volume of starships should be expected to be in ports.  So, on the
 order of half a dozen freighters of 50-200 dton cargo capacity,
 and/or one or two larger ships.

 That's also assuming it's only an endpoint of trade
 routes, and that no freight goes through the system.  In general this is
 usually a very poor assumption since most systems lie between many other
 pairs of systems and freighters must stop to refuel along their
 routes.  So we should probably multiply that by a factor of at least 2-10
 to account for this.

 Of course, there will also be smaller worlds and also those
 which have other factors then merely economic ones restricting their
 trade.

 - Tim