Frankly it's already 21GB and only has a handful of ships and locations compared the whole game
Basically, if you stick textures designed to be viewed in 4k, then you are going to take up a lot of space
Also given that such a vast amount of content is going to be included, but not randomly generated, I'm not surprised it's only going to keep growing. They do just need to focus on improving their download speeds, because patches already seem to take all day despite my 120Mb/s internet connection