The system data on my MacBook Air M4 takes up 128GB out of 512GB

System Data is mainly your own data, stored in the hidden Library and ~/Library folders. Examples include Outlook email attachments, local cache files for cloud storage, etc. Worth noting that System Settings > Storage has several categories, but anything that macOS doesn't associate with one is counted as 'System Data'. I also don't find it very reliable.

For example on my work Mac, System Data is 112 GB, about 40 GB of that is Outlook attachments. Applications is reported as 283 GB, but 200 GB of that is allocated local copies of our cloud storage solution, i.e. it's documents (powerpoint files, mostly) that macOS doesn't see that way.

You can use apps like OmniDiskSweeper or Disk Inventory X to view full disk contents by size (you'll have to allow access to various folders/locations on your Mac).

My M4 Pro Macbook Pro 14, in continuous use since March, shows this:

Your issue come sup often and, in addition to the excellent responses by others, I seen posts the suggest temporary print files, aftermarket anti-virus apps, "cleaning" software, and even Google temp files causing what you describe. I don't do any of the stuff. I use Office 365 but not Outloook.

Sometimes a Safe Mode boot can reduce the clutter somewhat, but not completely. See:

Start up your Mac in safe mode - Apple Support

I find letting the computer "soak"in Safe Mode for 15-20 minutes so it can complete its tasks seems to help.

Easy enough to try.

Hi Kotik3232

Approximately 100GB takes MacOS with system data...

Thats why the lowest storage that you can buy now is 256GB... Just ignore it... If you dont have the disk full its ok.

If you have it full just buy an external SSD Disk like me 😉 (I recommend Kingston XS2000 1TB)

neuroanatomist wrote:

You can use apps like OmniDiskSweeper or Disk Inventory X to view full disk contents by size (you'll have to allow access to various folders/locations on your Mac).

Thanks for chiming in!

Kotik3232 wrote:

Hi! I ran into this problem that the system data on my Mac takes up 128GB out of 512GB, although I bought it literally yesterday and didn't even have time to use it properly. MacOS takes up the standard 24GB

That seems like a lot. I have a two-year-old MBP that has only about 68 GB of system data.

Have you transferred data from a previous Mac? Or loaded a Time Machine backup? Are you in the process of syncing anything to iCloud?

Kotik3232 wrote:

I don't use iCloud sync and my Mac is new. I switched from Windows and didn't have these issues, so I was wary. Apple support sent me to their website because they don't offer phone support in my country. I don't use Time Machine either

This is site is not Apple Support. This is a user-to-user forum.

Hopefully someone with more knowledge will see the thread and respond. I don't think what you're seeing is normal.