In System Settings, General, Storage, macOS groups everything it cannot put in another category under System Data. It is not one folder, which is why it is hard to reduce.
What is usually in it
- Caches from apps and from macOS itself.
- Time Machine local snapshots, kept on the disk until the next backup completes.
- Logs and diagnostic reports.
- Virtual machine and container disks, for example from Docker.
- Developer data, such as Xcode’s build output and simulators.
- Purgeable space that macOS frees on its own when it needs room.
Why it looks larger than it is
Part of System Data is purgeable: macOS releases it automatically when space runs low. A scan that measures real folders will often find less than the Storage panel reports, and the difference is mostly purgeable space and local snapshots.
What you can safely reduce
- Clear caches for apps you no longer use.
- Remove old Xcode simulators and build output if you develop for Apple platforms.
- Reclaim space from container and virtual machine tools from inside those tools.
- Let a Time Machine backup finish, after which local snapshots are released.
Avoid deleting folders under /System or /private/var by hand. macOS manages them, and
removing them can break updates or apps.
