When backing up your catalogs on macOS, Lightroom CC 2015.1/Lightroom 6.1 and later versions don't compress catalogs that are larger than 4 GB. This restriction was imposed in Lightroom (versions listed below) because the default macOS unzip utility is unable to handle archives larger than 4 GB. It reports such compressed catalogs as 'corrupted' when you try to extract them.
The following Lightroom versions do not zip catalogs larger than 4 GB:
Catalog compression in Lightroom CC 2015.8/Lightroom 6.8
Beginning with this release, Lightroom compresses all the catalogs when creating a back up — including the ones that are larger than 4 GB.
Extracting compressed catalogs larger than 4 GB on macOS
To extract such compressed catalog back up files on macOS, use third-party zip utilities like 7-zip, StuffItExpander, or any other utility that can handle extracting large compressed files.
In Lightroom CC 2015.8/Lightroom 6.8 (Mac), the size restriction for compressing a catalog has been removed. Compressing the catalog reduces the disk space consumed by the backup and also improves the backup time, especially when the backup destination is on a NAS (Network Attached Storage).