That figure was provided by Sean Murray, founder of No Man's Sky developer Hello Games and a programmer. Posting on Twitter, Murray explained much of that 6 GB is actually taken up by audio.
For those asking - size of No Man's Sky on disc is just 6GB. A lot of that is audio (⌐■_■)— Sean Murray (@NoMansSky) July 10, 2016
This is likely achieved thanks to the fact that much of the content in No Man's Sky is procedurally generated, which means there's less space taken up on disc. Or it could all be witchcraft.
Continuing, Murray revealed on Twitter that No Man's Sky had passed certification in Europe, United States, and Asia. It is currently being submitted for certification in Japan.
Additionally, the studio is "already busy on Update 1" for the space exploration game. Details on what it will include were not discussed.
No Man's Sky went gold on July 7 and Hello Games celebrated with a selfie.
No Man's Sky is scheduled to launch on August 9 in North America, followed by August 10 in Europe, and August 12 in the UK. Hello Games recently settled a three-year legal battle over the No Man's Sky name, allowing the title to stay the same.
For more on No Man's Sky, you can check out GameSpot's "Making Sense of No Man's Sky's Massive Universe" feature.