Original Patch Size = 46gb Part that needs to be downloaded and updated on the Original Patch = 20gb Amount of space PS4 needs to download that part = 92gb (46x2) (PS4 needs to reserve double the size of the original patch due to how the storage system works on the platform) Size of Patch after the update = Usually the same, a bit higher when there is a new DLC on ESO.
I've had 30gb of space left on my PS4 one time and Overwatch or Hitman didn't download their updates because of everything I explained above. ESO is more apparent because it already had a big patch file since the beggining. Sony is to blame, not ZOS. Xbox One has a different file system and this problem doesn't happen there.