The way the planet generation system works - in both games! - doesn't really have any connection between number of systems and size of install anyway.
The original 1984 Elite fit entirely into 32k (i.e. 32,768 bytes) of memory and had 2,048 systems ... but with a very small modification to its source code it could have had 72,057,594,037,927,936 systems instead (and apparently some early prototypes of it did)
It would (counter-intuitively, but true!) take more disk space to only include the previously-explored systems than it would to include all of them; your ED install already includes the system data for the entire galaxy - or rather, it includes the mechanism to near-instantly generate the system data for any bit of the galaxy at any time, so quickly that you don't notice it wasn't there all along.