Why your Mac cleaner promises more space than it frees

You run a disk cleaner. It finds 40 GB of “junk”. You delete it, check your free space, and gain a fraction of that. Nothing went wrong with the deletion. The number was wrong before you started.

The short version

Modern Macs use the APFS file system. APFS can clone files: when a file is copied on the same volume, the copy can point to the same blocks on disk instead of duplicating them. Both files report their full size, but the data is stored once.

Most cleaners, and the classic du command, add up the logical size of each file. With clones, that total counts shared data several times. Delete one copy and the shared blocks stay on disk, because the other copy still uses them. You free almost nothing.

Where clones come from

You create clones without knowing it:

• Duplicating files in Finder on the same volume.
• App updates and code signing, where an updated app bundle shares most of its data with the previous one.
• Developer tools and package managers that copy large folders.
• Messaging apps that store the same media in more than one place.

The more you work with big folders, the bigger the gap between “claimed” and “freed” gets.

Other reasons the numbers disagree

Clones are not the only cause:

• Local snapshots. Time Machine and system updates can keep APFS snapshots. Deleted data that a snapshot still references is not freed until the snapshot goes.
• Purgeable space. macOS marks some caches as purgeable and reports them differently in different places.
• Files in use. A cache that a running app holds open may not be released until the app quits.

How to measure what you really get back

The only honest measure is the change in actual free space, before and after:

1. Note the volume’s free space.
2. Delete one category.
3. Note free space again. The difference is what you truly recovered.

Do that per category and a pattern appears: some caches free almost exactly their size, while cloned media and app bundles free a small fraction.

Doing it safely, automatically

ZwayStorage Warden does exactly this. It shows every cleanable category with its claimed size and a calibrated estimate of what you will really recover, based on the before-and-after measurements of your past cleanups. It gets more accurate the more you use it.

It also refuses to touch what matters. Documents, Desktop, Photos, message databases and system folders are on a protected list that can never be deleted, and every path is re-checked at the moment of deletion. It runs entirely on your Mac.