When "write changes to XMP/ACR" is activated, the heavy edits stored in the .acr file should not be duplicated in the .lrcat-data file. This would significantly reduce the size of that file and, by extension, the overall size of the catalog.

Since v15.0 of Lightroom Classic, users have been able to activate “write changes to sidecar XMP/ACR files.” According to Adobe, activating the creation of sidecar files will significantly reduce the size of the catalog. I have not found this to be the case.

I recently performed a test to measure the change to the catalog size when sidecars are activated. I created a test catalog and imported 108 Nikon NEF raw files. I then applied AI Denoise/Raw Details to one photo, and synced this setting with the other 107 photos. I closed the catalog and went into Windows Explorer to check on its properties. The .lrcat file was 5.10 MB in size and the .lrcat-data file was 1.38 GB in size.

I then opened the catalog and checked the box to activate writing sidecar files. I then clicked on the Metadata menu option and selected “Save metadata to file”. I waited for the progress bar to complete, checked the image folder to confirm XMP and ACR files had been created and closed the catalog. Returning to Explorer, I checked the file sizes and found the .lrcat and .lrcat-data files were the same size as before. They were not reduced in size.

Earlier today, I opened that test catalog and ran optimization on it. Per the attached screenshot, the .lrcat-data file remains unchanged in size. The .lrcat file is reduced about 9% in size.

Given that the .lrcat-data file is typically much larger than the catalog file, I request that Lightroom Classic remove the data stored in ACR file from the .lrcat-data file. This would significantly reduce the total size of catalogs in which writing changes to sidecar files has been activated.