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I'm looking at the size of a folder in get info, and Finder is giving me two different sizes, differing by about 10 GB! I'm curious as to the reason and source of this size difference. In the image below, it appears to be giving both a size of 43.51 GB and 53.24 GB.

same folder, different sizes

If this is useful, I'm running Catalina 10.15.7.

Thank you!

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  • I don't know enough about this to provide an answer, but some interesting info. I copied some folders from one hard drive to another. The copy came to half the size on disk as the original! I have no idea why, I can only guess that when it's copied, macOS finds the most efficient way to store the new copy, whereas the original was probably pieced together over a long time (adding a few files at a time) and hence was stored much less efficiently.
    – stevec
    Commented Feb 10 at 12:56

2 Answers 2

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The size of a file is amount of data (in bytes) that it contains. The size of a folder is the total size of all its files. But the space consumed on disk is nearly always a bit different and can be considerably different.

Some reasons for disk space being larger than file size:

  • Small files consume 4K (most obvious in programming project files);
  • file extended attributes.

Some reasons for disk space being smaller than file size:

  • File compression;
  • Sparse files which don't use disk for empty blocks.

Some reasons for disk space being smaller than folder size:

  • Duplicates of files within a folder, share the same space on disk;
  • Clones where parts of a file may share space with other files in the folder.

Most of the time we should not worry about the difference.

But clones and duplicates become an issue when copying files and folders between file systems as each file will now consume its full disk space on the destination.

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  • Extended file attributes? For example, does a "com.apple.quarantine" flag pull as much as that there is a noticeable difference?
    – BabyBoy
    Commented Apr 11, 2021 at 20:19
  • @DarwinOSX Noticeable? Depends how hard you look. xattrs are just one of many things that make a small difference to the reported sizes. I suspect my lists are incomplete.
    – Gilby
    Commented Apr 11, 2021 at 22:55
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The size value is the actual size of the uncompressed files while the 'on disk' value is the amount of disk space needed to store the files.

From https://superuser.com/questions/599112/mac-os-x-finder-substantial-disparity-between-size-and-on-disk-values-for-f the file size is the actual size of the data in the file while the 'on disk' size is the size of the actual disk space used by the file.

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  • I don't think compression is the main factor in this question where the disk space is much larger than the data size. Compression goes the opposite way and is just one of many factors contributing to disk space.
    – Gilby
    Commented Apr 11, 2021 at 22:59

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