Storage capacity puzzle

[omeletpants]

I have a brand new iMac 27" running Big Sur 11.6.1 The drive is a 250 SSD. When I checked the available storage it said I had 22GB available. So, I deleted a few applications, some photos (then deleted them from the recently deleted folder), some music, some apps then I cleaned out trash and rebooted. I thought I would have regained about 6 GB of storage, but in fact my available storage dropped from 22gb to 17gb. How can this be?

 

[ZebedeeG]

Okay, I'm no expert! But are you checking your storage capacity in Disk Utility or About This Mac?

When deleting stuff macOS may not have actually removed it from the disk, even if you empty the trash afterwards, so Disk Utility won't show the difference. In fact if macOS has been busy doing stuff it may have filled up further with temp files.

But "About This Mac" will show the free disk space actually available to macOS by including the stuff which it can delete when more space is needed...

At least this is my understanding - and I trust that if I've got the wrong end of the stick entirely one of the more techie types on here will jump in and correct me! ?

 

[omeletpants]

Okay, I'm no expert! But are you checking your storage capacity in Disk Utility or About This Mac?

When deleting stuff macOS may not have actually removed it from the disk, even if you empty the trash afterwards, so Disk Utility won't show the difference. In fact if macOS has been busy doing stuff it may have filled up further with temp files.

But "About This Mac" will show the free disk space actually available to macOS by including the stuff which it can delete when more space is needed...

At least this is my understanding - and I trust that if I've got the wrong end of the stick entirely one of the more techie types on here will jump in and correct me! ?

So, I asked this same question on Appleinsider forum and was told that it takes awhile for the available space to register. Went back the next day on "about this mac" and it finally registered correctly. Now says I have over 50gb available instead of 22gb. You would think it would register immedaitely

Thanks for your help

 

[wilberforce]

MacOS APFS file system supports clones, sparse files and transparent compression, so physical file sizes (actual number of bytes occupied on the SSD) are not the same as logical file sizes (as reported by MacOS).
So the math almost never adds up.
For example, if you duplicate a lot of files (which creates clones, sharing data blocks with the original cloned files), it is quite possible to have a folder that is larger in size than the SSD. I have had a 1.2TB folder stored on a 512GB SSD. When I go to the storage folder in "About this Mac" the reported size of my documents is larger than the total SSD size, but it still says some of the SSD is available.
So if you delete a cloned, sparse or compressed file, the actual amount of SSD storage released will be much smaller. If you modify cloned files, they will occupy more space (because they can share fewer data blocks).

However, I can't help explain why deleting files would result in less storage available