what is PCG, it is just a alternative where the assets come from.
It can be somewhat more than that - essentially it can be a trade-off between memory use and CPU use.
Consider a small planet, only 1000km in radius. It has a surface area of 12.5
million square kilometres (plus a little bit more since it's not a perfect sphere, but close enough). To require a pixel per square kilometre to avoid a blurry texture, you only need to be within 2000km or so of the surface.
So, that's - rounding up to the nearest power of 2 - a 4096x4096 texture, or 67Mb of texture. From "low earth orbit" - 100km above the surface - you'd need 20 times as many pixels in each direction. With a bit of work, you could probably get away with a 65536x65536 texture (or 16Gb of texture file) which of course you'd split into multiple textures so you didn't load into memory the bits for the side of the planet the player couldn't see or couldn't see at that level of detail.
Now, you need one of those for every planet or moon in the system, and there's currently 55 systems, and some of the planets are gas giants. So, that's getting on for 100 terabytes of download just for Beta 1, and that's not even got the animated atmospheres, or city lights at night, or volcanoes erupting, etc. that the full game is supposed to have: that's just for a static colour map for each planet. (And it'd look terrible when it's time to actually land on the planet later - the pixels are still bigger than your ship)
... or, you can use some clever shaders and (possibly) some small textures as source data, and get an entire galaxy's worth of unique planets. It requires more processing speed than a basic UV-mapped sphere, but the recommendations for CPU and GPU processing power are - compared with the memory requirement - quite substantial.
...and that means that the contents of the main-memory resource cache can be far smaller than originally seems possible, because that's
not the final representation of the texture. That never even exists all in one place - bits of it are calculated and discarded on the GPU every frame as necessary.