Put into perspective, take a gander at the cargo tables in the KH book. A unit of jetcopters costs 30K at the source and 75K at destination.

Now keep in mind, Jetcopter Jim's dealership isn't going to pay full retail to have copters shipped from their source. He has to buy that 75K unit of jetcopters at a price where he can make a profit on them when he sells them at his dealership. Remember, he also has to pay to ship the copters from the space station to the surface once the freighter drops them off...

Your standard SF jetcopter is listed at 40K in the AD book. So that "unit" of jetcopters is easily comprised of at least three copters, each with a limited amount of "hold" area for stowage (500kg/5 cubic meters). Consider that the three+ copters takes up a full "unit" worth of space and the entire copter itself is not a cargo container, only a fraction of that copter volume is dedicated toward stowing cargo.

Further illustrating the shortcomings of a cargo "unit".

And IIRC, the cargo unit is only applicable to freighters which start out at HS:5. If a 10x2 meter HS:1 ship could contain one cargo unit, how many of those 10x2 ships could you fit into the Gullwind's cargo hold (20m diameter with an easy 40 meter height)? Yeah, you could park six to eight in a circle on the floor alone and have 6-8 cargo units capacity, with plenty of room to spare.

In that regard, my 1 unit capacity on the RT-3100 merchant scout (HS:3) is pretty generous.

Nay, a shuttle (to which I would designate as HS:2 minimum) would carry a single jetcopter at best, or half a unit in my game (which, again, is rather generous). On that note, back to "how many copters per unit"...how much would it cost to transport copters one at a time from station to surface?  The fuel "load" alone is 250CrxHS, and the shuttle owner won't fly it for free let alone at just the simple cost of fuel. You're easily looking at shelling out 1000Cr just to move a single copter from the station to the spaceport, and unless Jetcopter Jim has his own ground transports he will be paying to ship them from starport to dealership, after all if he flies them to his dealership they're no longer "new"...so three copters per unit is also generous, and the only reason I wouldn't make it four is the fact that my 0.5u shuttles would be able to ferry them two at a time, thus deflating their value at Jetcopter Jim's dealership 

I picked up a book from a game called "Shatterzone" a few years back and it has a similar system, albeit they utilize what is coined as "bays". You have engine bays, computer bays, weapon bays, crew bays, so on and so forth. Anything that wasn't utilized became a hold.

Again, I'll reference the Gullwind.

It has a cargo hold that is roughly 40% of the overall craft at best. Assuming standard KH dimensions, she would be 20m at her girth and 130m from stem to stern, or a total of (pi * r^2)*L = 40,820 cubic meters. 40% of that (the hold) comes out to 16,328 cubic meters, or 6 units by KH definitions.

Now apply that very same math to a HS:18 freighter...85m in diameter and 500m long or 2,835,813 cubic meters. Assuming a similar 40% allocation for its "18 unit hold", the hold capacity is 1,134,325 cubic meters.

Now look at the math --- 6 unit hold times a factor of three for the HS:18 vessel yields 18 units a la KH rules. Simple math says that the 40% space allocation of 40,820 cubic meters times three equals 122,460 cubes, just a hair over ten percent of the above example. For a HS:18 freighter to stick to the same parameters for "unit" measurements, the hold would be a much smaller ratio to its size, begging the question of just what will the ship's less-than-20 crew be doing with all that extra not-cargo space in the ship.