Transcendence

UCD is stored by the kilo in your hold, but is bought by the gram at Commonweath stations and is listed as grams in your hold.

Drug dealers and their clients are picky about these sorts of details
(wait... you just took a kilo of my dust, but only paid me for a gram. Now I kill your family) so it probably pays to get it right.

Perhaps that 1g of UCD is surrounded by 999g of packaging...

In reality it's because the kilo is the smallest unit of mass in Transcendence. In universe the stuff probably needs to be kept refrigerated which adds a lot to the mass.