That's the opposite actually. One "standard cargo capsule" with a "fixed volume" full of feathers as you said, would have much much less than "always 1 metric ton". In order to fill one ton you'd need much much more volume. But they do add one ton, as I said, as you can see in the ship statistics after picking up cargo in a station.just eject cargo and look at the capsule.
the "standard cargo capsule" (TM)
- has a fixed volume
- always weights 1 metric ton.
so, maybe, one capsule of feathers is not completely full, but at least limpets and cargo rack "racks" can be of standard size.