DiskWrightWhen a disk utility shows two size numbers, it is not necessarily being inconsistent. The numbers can describe different properties of the same file. Keeping them separate is useful when you are trying to understand a drive, especially before estimating what a cleanup will recover.
What logical size tells you
Logical size describes the file’s length: the amount of data it presents to an application. It helps answer questions such as how large a document or export is from the application’s point of view.
That is useful information, but it is not a promise about how much physical storage belongs exclusively to that file.
What allocated size tells you
Allocated size describes storage attributed to the file by the filesystem information available to the tool. The exact meaning and availability of those measurements depend on the filesystem and the APIs used to read it.
DiskWright keeps logical and allocated measurements separate. When allocation information is unavailable, the result should remain identified as an estimate rather than presented with false precision.
Why the difference needs interpretation
Compression, sparse allocation, shared storage and cloud-backed files can complicate the relationship between visible file length and storage consumption. A difference between two size measurements does not, by itself, identify a single cause.
For the same reason, subtracting “size on disk” from “size” is not a reliable way to claim exact compression savings. That would require evidence about the file’s actual storage representation, not just two totals.
Why deletion may recover a different amount
A cleanup estimate is a forecast. It must account for what the selected operation actually does and what storage can be reclaimed afterward. Moving a file to Trash, for example, changes its location before you make a separate decision to empty Trash.
Treat an item’s measured size as a way to prioritize inspection. Treat observed free space after the completed cleanup as the result. Those are related measurements, but they are not interchangeable.
How we present this in DiskWright
The inspector shows the two measurements side by side instead of compressing them into a single unexplained value. It also ranks the largest items inside a selected folder, so you can keep investigating before staging anything for cleanup.
Our rule is straightforward: a tool should explain what it measured, identify uncertainty, and avoid turning an estimate into a promise. That makes the numbers more useful, even when the answer is less tidy.
Continue exploring
This article describes the distinction used in DiskWright’s interface. It does not claim exact APFS compression or unique-block accounting.
Written by the DiskWright team. Practical guidance for understanding your Mac’s storage.
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