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!"Except that, even just passing through requires traffic control, some extra capacity for tugs and rescue just in case, and refueling. Figure maybe 10% added capacity for every multiple of normal traffic going to/from the mainworld. So if (just spitting random numbers) 10 ships a day go to/from Myanmarus IV, and 33 ships a day just pass through, then you need to add 3.3X10% to your system infrastructure, and there's probably Imp laws (in the 3I, and similar in some other major polities) that make you round up, not nearest, so Myanmarus IV has to expand traffic control, Search & Rescue, refueling, System Pilots & Tugs, and yard capacity by 40% to account for the ships just passing through, even if they pass through the outer system and frontier refuel from a gas giant (assuming Myanmarus doesn't have a station out there to collect fees for skimming from their gas giant).