total file size concerns

i have been having some issues recently with my install of PoE2, after the 0.4 update my steam client was trying to install multiple versions of the game for some reason, I moved my install off of my SSD onto a storage HDD that I have some of my older games saved to, it seemed to fix the issue, however it was noticeable in game that performance is absolutely terrible (load times of minutes on just getting into the game, several minutes transitioning between zones and a record of 12:17 just to be able to interact with the atlas map).

i decided to move my install back to my SSD (which had over 250GB of free space), and after completing the process it left my drive with 10gb of free space, so my question is, why the hell is the PoE2 install taking up basically TWICE the total file size on drive?