I'm confused about difference in disk space with Lightroom Classic

Are file sizes a different size once they are edited?

By @Kenny Ingle

Not directly, but definitely indirectly. This can be a little confusing, so hang on.

Editing a raw file in any parametric editor such as Lightroom Classic (but also in other raw apps) doesn’t change the file size of the original files. So if you import the same originals to your primary (working) and secondary (backup) drives, regardless of how you edit, the size of the two sets of originals should remain the same.

There are some exceptions; for example if you apply an edit that results in a new file then of course that file doesn’t exist in the other set until they’re mirrored. Some examples of edits that add new files are Enhance (including Denoise), and merges such as HDR or Panorama.

So if normal edits don’t change the size of originals, where are the changes stored and how is space consumed? This is where you might get some answers. Suppose you just imported a large set of files to both your primary and secondary drives. Here is what is going to happen next.

1. Actual file import. Identical space usage on both drives. 

2. Preview generation. Depending on how you set up Import settings, Lightroom Classic may use some or a lot of space to store previews generated for the imports. Regardless of the settings, the previews will be stored in the same folder as the catalog (in the “...previews.lrdata” file). For example, if the importing catalog is on the primary drive, then the generated previews for that import are on the primary drive only.  Best case scenario: You set File Handling/Build Previews to “Embedded & Sidecar” so that Lightroom Classic previews are only generated on demand or while editing in the Develop module. Worst case storage scenario: You set it to “1:1” so Lightroom Classic will build a full size preview for every image imported, possibly increasing the size of the previews file by a lot with every import. Extra worst case scenario: In Catalog Settings/Previews, the new option “Limit Preview Cache Size” option is not being used, so the previews file can grow until all storage is used. (My previews file has exceeded 30GB at times.)

3. Edits. All edits you make are stored in that catalog. (In other words if the same image was imported into two catalogs and edited in each, each catalog stores its own edits for that image.) Simple edits like changing Exposure use very little space because they’re just database entries in the catalog file. But edits that generate masks may increase the catalog size more. The point is, like previews, these increase space usage in the folder containing the catalog.

4. Camera Raw cache. Work in the Develop module is cached in the Camera Raw cache to enhance performance. You can control the size limit of this cache in Lightroom Classic Preferences/Performance. By default, it’s on the boot volume, but it can be relocated using one of those options.

5. Video cache. Also in Lightroom Classic Preferences/Performance is the Video Cache setting. If you view a lot of videos in a catalog and this option’s limit is not applied, the video cache can consume a lot of storage space. It looks like this cache cannot be moved, so it can increase space usage on the startup volume.

Now, putting all of that together:

If your catalog is on your primary drive, then all space consumption due to edits is going to happen on that drive. Not because the original files are getting bigger with edits; they aren’t. It’s because your catalog is on that drive, so it is in that folder that you might see significant usage of storage for the growth of both the catalog size and, even more, the total space used by image and video previews.

In addition, if your primary drive is also your startup drive, then as you edit, the Camera Raw cache and Video Cache will increase unless limited. So storage gets used in multiple places on the startup drive.

Now, all of that is pretty common, a lot of people edit from a catalog stored on their startup volume, so many Lightroom Classic users see space consumption happen more or less as described above.

I have a laptop with limited space, so I chose to reduce how quickly space is consumed in two ways: I set my import Build Previews setting to Embedded & Sidecar, and I set a limit in the Limit Preview Cache Size option and the Limit Video Cache Size option.

Of course the other option is to move the catalog to another volume. Then all of the space usage would happen on that volume instead. (Except for the Camera Raw cache, unless it’s relocated away from the startup volume).

When I import files the files go to both.

Do you mean that the option in the LrC Import dialog box to make a second copy has been chosen? Or do you mean something else?

What disk is your catalog stored on? What disk are the catalog backups stored on?

What else is on your primary drive?