It's all procedurally generated so it's not taking up space on your HDD. The core assets are there for art, user interface and programming stuff for the game to work.
But as far as maps, it's all generated on the fly and calculated mathematically using the systems that NMS uses for terrain, creature, and world generation.
For example Minecraft is a very small game, only a couple hundred megs at best, but it's world sizes are near limitless compared to preset world assets in open world games like Skyrim, The Witcher 3, Fallout 4, and several others.
In fact there was an article that someone walked in a straight line in a Minecraft world for 3 straight years and only traveled apparently 700 or so kilometres. A Minecraft world you spawn at it's center from the center to one end it's roughly 12,000km. (At least in the version of Minecraft he played which is beta 1.7.2 which future versions this Far-End thing was patched out)
http://kotaku.com/a-man-has-spent-three-years-trying-to-walk-to-the-end-o-1509473657
Outside of static assets like music, art, textures, and game code. NMS won't take much space.
In short: World size has no bearing on how much disc space is used, especially in regards to procedural generation.