i had this problem too, and i reached out to apple support:
Normally system storage is that which is stored in the library of the Mac.
The next thing to do will be to clear all Caches. Caches are like bookmarks. Where it allows you to find sites or files faster. Caches does the same thing for applications.
Caches are stored in the hard drive and not on the memory. However, Caches run from the Memory and impact the performance on it. This will not impact or delete any personal information.
To do that I will need you to go to Finder. Once in Finder, on the top left hand corner of your display there is an dropdown option named “GO”. Please click on there.
On your keyboard please hold down your OPTION key.
The library folder should populate in the dropdown you had selected. Please select the folder named LIBRARY.
Once in the library please click on the CACHES folder and delete everything in there.
Now that this is done Open Finder and select the “Go” menu at the top of the screen. In the drop down, select “Home”
With the Home folder option, switch to List View (Command-2) and open “Show View Options” from the View menu (Command-J)
Place a check in the checkbox for “Calculate all sizes”. You should start seeing each folder calculate the size in Finder. The bigger ones may take longer
This will help you determine what folder is using the most data. Next, go back to the “Go” menu in the menu bar at the top of the screen.
With the “Go” menu open, hold the Option key down and click “Library” when it appears.
With the Library open, pull up the View Options menu (Command-J) and place a check in the checkbox for “Calculate all sizes”. Check what is using the most space. Application Support may be taking up a lot of space if multiple 3rd party applications are installed.Check each folder and subfolders within Users to see the culprit of what is using the most Space. deleting the whole folder will be just fine as long as you are confident you don’t use or have deleted them.Any application that you don’t use will be fine to delete as that won’t impact the performance on your Mac or the Apps.
The Containers folder categorize is normally created by third party programs that are made to scan the computer. Certainly not pertaining to the Mac, I would suggest to erase it.
I was able to clear up 84 GB with these tips. Also - be sure to erase iMessages in that folder because that was where my bulk was.
