The question here might be why do system data use so much storage? In the macOS system, there is something called "System Data" which contains several kinds of files that don't belong to Applications, Photos, and Documents. They may contain things like caches, logs, temporary files, application support files, system resources, or local snapshots. It depends on how you use your computer whether it gets increased or decreased. Check the System Settings > General > Storage to see your storage usage. When you notice that System Data is too big, delete any unnecessary files from other categories, old backups, and reboot the Mac.