Why is System Data and macOS on MacBook Pro consuming a lot of storage space?

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System Data technically, has a mount point /System/Volume/Data 

With a mount point of /System , this Volume is part of the Operating System and can not be deleted - full stop

It is where your User Account (s)  was made when the computer was First Setup as New

We can point-out what may be included in this Volume

Though, how to reduce this is a long and complicated trial and error process

The category is a collection of files which include:

• System temporary files

• macOS system folders

• Archives and disk images (.zip, .iso, etc. - often found in the Downloads folder)

• Personal user data

• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)

• Cache files

• Fonts, plugins, extensions

• Other files that are not recognized by a Spotlight search

• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package

•Time Machine Backup Snapshots that have not been transferred to the TM Backup Drive 

Locate backups of your iPhone, iPad, and iPod touch.

•Was there or is there another user account currently or previous used on this computer ?

Has the account been removed ?

Did the removed user account ’s Home Folder also been removed ?

Delete a user or group on Mac

• If using a Cloud Based Services to sync files,  like iCloud and One Drive. These can also occupy space on the Internal Drive 

They can be located anywhere on your hard drive. The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use either of these two free apps, GrandPerspective , to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.  

Another Application to find and analyze what is using space on the drive is Etrecheck >> Clean up

Thank you for the additional information

The really issue is, having a small 256 GB Drive Capacity Drive in 2025

I thank @steve626 for also mentioning this fact

We get what we paid for - 256 GB SSD drive

It is akin to when Apple sold computer, back in the day, with the tiny 128 GB SSD Drive

Less expensive than the costs for a larger drive that would have Future-Proofed to computer to meet current and Future Needs for Storage

That said you can only manage the space that is available, 256 GB SSD as best as you can

These are those who believe Starting Over from Scratch is the quickest and most efficient means to solving this issue

Personally, I have some issues with this approach only because if the User does not understand the how, what and why this is happening.

Most probably they are prone to repeat the same errors again and arrive back here is some period of time with exactly the same issue again

Since you mentioned earlier that you have never used Time Machine Backup before

That would the the Starting point before moving forwards

Always make a Time Machine Backup  before doing anything or some other way to backup up to a reliable External Drive

This Backup will be required now and in to future

It is one of the few ways to protect you Non Replaceable Personal files

Starting over from Scratch will depend on if the computer is Apple Silicon of Apple Intel based computer

For Apple Silicon computer >> Use Disk Utility to erase a Mac with Apple silicon.

For Apple Intel computers >>   Use Disk Utility to erase an Intel-based Mac followed by How to reinstall macOS

To reiterate, Always make a Time Machine Backup  before  proceeding 

Once done Setup Assist should be presented

I would suggest Only Migrating the User Account and nothing more.

Reinstall all your Needed not Wanted third party application also from scratch

I can see you improved your situation, but there are still issues:

Your System Data was reduced by 15 GB, probably be deleting some Time Machine snapshots, which in fact you can do yourself in Disk Utility;

Your Applications was reduced by ~ 2.5 GB, how was that accomplished;

My guess is that some cache files were also removed. However all of the above (except maybe the Applications files that were deleted) will likely be recreated, maybe slowly, maybe quickly.

However your free space is still less than 10% and this will likely impair efficient operation of the computer. A good rule of thumb is at least 20% free space at all times.

Some of the contents of "System Data" include various files and folders that are associated with some of the Applications. Some of these are found in your user Library folder, inside folders such as Application Support, Preferences, and others.

Onyx has been around for a long time. I don't use it but it many others have. It can help with clearing out some files but the effect may be temporary, many will be recreated. Some of the files it can remove, such as certain foreign language files, will be recreated at the next system update. The danger with software that primarily deletes files is that something important for a smooth operating system will be deleted along the way.

I think the real challenge is that 245 GB is such a small internal drive that keeping 20% free space will be a formidable challenge. This will never go away; as you use the computer more you will use more space.

To really get to a good "audit" of exactly what is taking up your space, use a utility like Daisy Disk, it will show EVERYTHING. BE sure to "scan as administrator" to see and catch all the hidden or protected directories.

Some advocate Start Over from Scratch

Meaning to wipe the drive of everything

Reinstall the operating system

Reinstall all the Third Party Applications that are needed, not wanted

This will basically present an " Out of the Box experience "

This will also solve the Short-Term issue though will not solve the Long-Term issue which has been mentioned several times my several different Contributors

For Apple Intel computers >>   Use Disk Utility to erase an Intel-based Mac followed by How to reinstall macOS

Always make a Time Machine Backup  before  proceeding 

It would be suggested to Only Migrating the User Account and nothing more.

Apple Beta — Restore Shipping Software

I rather think it is a bug from Sequoia as this inflated System Data was not present with previous OS. Data system is taking now up to500Gb (half of my 1Tb SSD)I have the same issue on 2 different machines with Sequoia 15.4 (Macbookair and Studio). I have updated to 15.5 and still the same issue.

Your numbers are not that unusual. I have 25 GB for MacOS and 100 GB for System Data.

You might need to try to trim your Messages, are they in the iCloud? You can delete older Messages. Your Mac has very little storage, you have the model with the smallest amount of storage that Apple offers.

DStrivay wrote:

I rather think it is a bug from Sequoia as this inflated System Data was not present with previous OS. Data system is taking now up to500Gb (half of my 1Tb SSD)I have the same issue on 2 different machines with Sequoia 15.4 (Macbookair and Studio). I have updated to 15.5 and still the same issue.

Speculative at best

If I were to use the Same Reporting Method that OP used

The reported values, for System Data on 1 M2, 1 M3 and 1 M4 machines all report about the same usages

Great to hear, I’ve never had a problem using Onyx to clear everything & the rest it does.

Hello, I have exactly the same problem on my MacBook Air and on my Studio Mac. It is taking up to 500Gb of disk space on 1 Tb disk. I have updated to the last version of Sequoia but it did not fix the issue.

I would appreciate any help from Apple.

I am sorry but you originally mentioned Sequoia 15.4

In the same time mentioned signing up for Beta Testing

Now it appears there is a retraction to this

If you have indeed unrolled the computer from Beta Testing

We need a very clear answer Before we can move towards with this issue

+1 on the less than 10%, in fact I consider that very dangerous.✌🏼