Somebody's living in a fantasy world.
A ship with 3x the payload of a T9 or Cutter better
not have a hull that's half (or, in the case of the Cutter, a third) of the mass. [where is it]
Let's take a look at this a bit more closely though.
If a ship weighs, say, 100t and has a maximum payload of 100t of cargo, we could call that a "payload factor" of 1 (cargo mass / hull mass).
If a ship weighs 100t and has a maximum payload of 50t of cargo, it'd have a "payload factor" of 0.5.
If it weighs 100t and has a maximum payload of 150t it'd have a "payload factor" of 1.5.
So, bigger number is better.
Currently, every ship in the game has a Payload factor ranging between 0.2 (The DBX - heavy hull but small payload capacity) and 1.6 (surprisingly, the Hauler - light hull and decent payload capacity).
Even the magical Anaconda only comes in with a Payload Factor of 1.2 (400t hull and 468t of cargo capacity).
The dedicated freight ships in the game are fairly average in this regard.
T6: 0.7
T7: 0.7
T9: 0.6
Keelback: 0.5
T10: 0.4
If the Panther Clipper could carry 2,100t of cargo
and had a hull-mass of 400t it'd have a Payload Factor of 5.25!

It'd be at least 3x more efficient than
any other ship in the game and it'd be fully 7x more efficient than any
cargo ship in the game.
Based on the stat's of other ships in the game, a Panther Clipper capable of hauling 2,100t of stuff
should have a hull-mass of around 3000t which would mean it'd probably have a dry-weight of around 4000t or 6000t when fully-loaded.
Which does rather raise the interesting question of how fast it could move, and how far it could jump, using existing modules
or what size of modules would be required to make it viable.