Start by checking Storage in System Settings, then compare likely folders over time. A changing free-space number alone won’t tell you which files caused it.
Start with the storage overview.
Open System Settings → General → Storage. Let the categories finish loading, then note your available space and the largest categories. Apple’s overview gives you a useful starting point for deciding where to look.
If you see System Data, treat it as a broad category. It includes files that don’t fit into the other categories; the label alone won’t identify a particular folder or explain a sudden change.
Keep a short note of the reading and the time. A second reading after your usual work can help you distinguish a one-off change from a pattern.
Follow the folders that grow.
Start with locations you recognize: Downloads, a video project, or a folder where an app saves exports. Consider what happened just before space decreased. Did you download a game, render a project, or save a second copy of a large file?
- Choose one likely folder and select File → Get Info in Finder.
- Record its size and wait for your next normal work session.
- Check the same folder again. Use the same folder and the same size reading so the comparison stays useful.
A larger folder is a clue. Open it to investigate which files changed before deciding what to remove. A smaller folder can also explain a rise in available space.
Understand what each total tells you.
Your folders’ file sizes and your Mac’s available capacity describe different things. Don’t expect a list of selected folders to account for the entire drive.
Backups can also keep earlier versions of data. If you need to inspect local snapshots, Apple documents how to view them in Disk Utility. That is a separate investigation from following changes in your everyday folders.
When you choose to remove something, check the Trash too. Keep anything you may need, and review Apple’s storage guidance for the built-in options.
Record what happens next.
If space keeps decreasing, repeated measurements are more useful than another isolated total. Track the folders you suspect, complete an initial check, and compare later results.
DiskEzy for Mac records additions, removals, and folder growth in the locations you choose. Its storage history helps you follow a daily change back to those folders. Allow the access macOS requests so the app can check your selected locations.
Measured history begins with the first completed check. It cannot recreate a complete record of changes from before tracking started, and it may not explain every part of macOS’s System Data category.
For a worked example, see how to track folder size changes over time.