Re: Lightroom Library.lrlibrary and Mobile Downloads.lrdata File sizes

Originals should drop out automatically although it does take time. As you say it is safe to delete  everything in the originals folder once it is confirmed all files are sync’d & backed up. It is possible to control the size of the cache in preferences. The default is 25% but you can make it lower and even set it to zero.

I'm having the same issue - so I went into the file Lightroom Library.lrlibrary, unpacked it and deleted everything in the Originals folder (once I was fully synced and backed up to the cloud).
This freed up approximately 40GB of data. I'm not sure why it stored pics in the Originals folder on my local HDD, considering I have turned off local storage of originals from within Lightroom CC, opting for cloud only storage. But this should not be happening, especially if you choose a small local cache size (one would assume).

Originals relate to new photos added to Lightroom. I'm not sure what the temp files are unless it was space required when migrating from another application such as Aperture or Lightroom Classic. 

Are you using the default cache or have you set preferences to store copies on a local or external hard drive?

I had this same question. Also, I am quite confused on which files--catalog, originals, previews etc. I can move onto a external hard drive. I'm not super familiar with how it all works, but lightroom is taking way too much space on my hard drive right now.