Ships.

 Ships can be transferred to cargo.
The cu size amount matches the stored ships cargohold.

(ex.) A Behemoths inventory
Local inventory
    Atlas mk1 (32cu)
    Atlas mk1 (32cu)
    Revenant mk1 (22cu)
    Etc.
So total space of the moth is (86/120cu) 

This allows people easier transport of ships.

Guild Hangars (inventory)

A guild Hangar that can be created by commanders or Lts.. When a guild inventory is created a key can be given to anyone in the guild trustworthy enought to use it properly. (A key kinda like conqstations.) 

Nationalist guilds can use it for shared storage of modules. Trade guilds can use it to transfer items between players without having to undock.
Shared inventory :)

It makes no sense at all for a ship's size to be based on it's cargo. That's obviously not the size of the ship. Ships are already assigned a size of 100cu, 200cu, or 300cu based on class. Light ships like EC, centurion, valkyrie, and vultures have 100cu. Warthogs, hornets, and revenants are 200cu. Behemoths, centaurs, and proms are 300cu.

Hence my title

Still, I think the point is being able to throw your ship onto a moth.
So you could transport 2 valks in an XC... cant transport Proms... :-/
But being able to load up a trident with some 'empty' proms would be cool... this ties into conflict diamonds post of persistent caps after captians log out, and having extra ships on board.