Cargo in the dedicated cargo hold of vessels with that feature have special encumbrance rules:
For listed equipment, there is a 10x packing rule (which I've seen proposed on various forums) for the dedicated cargo bay of light freighters.
The goods stored in the cargo hold are difficult to access at all (possibly requiring skill checks if the vehicle is moving), being safely locked in place and whatnot; in addition, if someone actually gets at the cargo they (or the ship itself) will be in danger due to the cargo moving around when the ship manoeuvres.
For a qualitative (narrative) description of the cargo capacity: it is about 1 cargo crate per 5pts of cargo encumbrance.
For a mass limit, one can use resources like wookiepedia,
this may be used to limit the number of crates of things like heavy machinery,
or raw materials.
The same encumbrance limit number constrains the amount of extra personal equipment that the crew/passengers can pack into the living portions of the ship. The 2x packing rule would apply, and this equipment would be relatively easy to access even when the vehicle is in motion (unlike the cargo in the dedicated cargo hold).
This last bullet implies that there is a separate "cargo hold" capacity from the "living quarters" storage capacity, and that are covered by different packing rules.
Finally, these rules only apply to vehicles designed for cargo, so, for example, they don't affect the capacity for a TIE fighter. However, in my quick check, they do seem to make sense for some of the land transport vehicles (that heavy truck like ground vehicle whose name I can't seem to remember), but wouldn't for a swoop.
Discussion for YT-1300
I'll use the holdout blaster as a representative item. If it's similar to a current day handgun, it fits inside something like a 10cm by 30cm by 50cm box,
and has a mass of about a kilogram (including box). Thus about 66 of them fit inside a cubic meter.
If we go by the rules as written a YT-1300 can carry 330 such pistols (packed) -- this works out to a volume of about 5.0 cubic meters, and a mass of 330kg. This is about the cargo capacity of a large cargo van.
Note that this is one way to resolve the apparent conflict: Han and Chewie are "two guys with a van" instead of having a vehicle that is capable of carrying industrially significant quantities of cargo. This approach leaves aside the issue that external sources rate the cargo capacities in 10s of tons for this type of ship, and it scales up to 1000s of tons for larger ships, which would not be reflected in these rules.
Using the 10x packing rule allows a YT-1300 to carry 1650 holdout blasters; this works out to a volume of about 25 cubic meters, and a mass of 1.65 tons.
This compares favourably with the 33 crates obtained by diving the encumbrance rating by 5 and assuming that each "crate" is slightly more than 1 cubic meter (especially considering that there is always wasted space in trying to pack cargo). Going the other way makes sense too: 1650 pistols in 33 crates works out to 50 guns per crate. Note that a volume of 33 cubic meters is about the size of a 20ft shipping container, so a YT-1300 would be somewhat like a large diesel truck or smallish tractor trailer. Note that heavy machinery or bulk metals could easily exceed densities of 100kg/m^3 and in those cases the mass constraint could come into play.
By the rules, a GR-75 has about 4.8x the capacity of a YT-1300, so something
like 160 crates. If you consider the underside of this transport you can estimate that that it has something like a few hundred (at most) containers attached to it -- so it is line with the qualitative "one crate per 5pts of encumbrance" rule. One can further scale up the effective capacity by ruling that for "medium freighters" a "crate" is larger (maybe 2-3 cubic meters?) and so on.
This means that it might be worth considering a 20x or more "packing rule" for listed equipment when packed into the cargo bay of a medium freighter. This kind of logic could scale up to heavy freighters and so on, but I haven't worked through many examples since so far, I've only worried about light freighters.