Good question! For the most part, the “Other” or “System” storage will store OS System files needed for the OS, however these typically aren’t above 20gb, but will differ depending on how you use your Mac.
Also, other apps may generate or store files that are categorized as “Other” or “System” because they are store in a hidden location that doesn’t have the standard title of “Documents” “Desktop” etc.
Many times it may also be other Users on your Mac as well.
You May have already come across Apple’s article:
How to free up storage space on your Mac:
https://support.apple.com/en-us/HT206996
If you have already and still can’t account for the large storage consumption, this may help:
As far as “Other” or “System goes, I’ve found in the past sometimes they are files you may not even need or want, here are some good spots to check, first I always recommend doing a Time Machine backup and/or iCloud or other similar backups:
First, the usual suspects, I’d check Finder:
On the top menu hit Go > Computer > Macintosh HD (Usually).
In there check Users to see if there is another user on your Mac you don’t need or want, I’d so delete in System Preferences > Users and Groups > Delete the home folder.
Also under Finder > Go > Home, I usually check for iCloud Drive Archive folders, which are backups of your iCloud documents, created when disabled. I’ve found that sometimes I see unnecessary duplicates of them and can delete them, (This won’t affect what you currently have in iCloud).
Also in there check folder sizes of other services like Dropbox, Google Drive, etc., you can highlight them and press Command i to see the size (On the top right of that window). And if you have that backed up, can clear them out if using too much storage.
OTHERWISE, the data stored maybe hidden, if so:
Open Finder.
On the top menu select Go.
While holding Option on the keyboard select “Library” under the Go menu, (Its hidden by default).
In the ~/Library, select each of these folders, then press Command I (i as in info). On the top right of the resulting window, it will show the size of the folder, we are looking for folders of large size:
Of the folders, check:
Application Support
Caches
Containers
Group Containers
Logs
Mail
Messages
Mobile Documents
Not all folders are just expendable, but store data or have some essential function for the OS or some other program you have.
If you find a large sized folder above, check:
~/Application Support/MobileSync/Backups, (These are iOS backups, perhaps you have old ones you don’t need anymore and can remove).
~/Caches, (If this folder is large, you can safely delete all the contents, although initially it may slow things down, they will build up again as you use your Mac, but you will not lose any data).
~/Containers/com.apple.mail, (If this folder is very large, most likely you have Mail logging enabled, and if it’s Mail logs using all your space, you can safely delete them and disable Mail logging to prevent this in the future):
Open the Mail app, On the top menu, select Window > Connection Doctor > Show Logs, once that Finder window opens, hit Command i again to get the size, of it is large, delete all the contents and empty the Trash.
You can them disable Mail logging in the Connection Doctor window by unchecking it.
~/Group Containers, (I’ve found this folder to hold important data, such as Office documents and more third party data, and some local data, if you find this folder to be larger than it should be, let me know).
~/Logs, (These are typically just diagnostic logs and can also be deleted if using too much storage).
~/Mail, (This folder does hold copies of all of your Mail, so is important, all though you may have an IMAP account, and they are safely stored on the server, your Mailboxes “On My Mac” are also here, so also wouldn’t want to delete them without that backup and/ or exporting them first. I have seen certain email providers cause a glitch to where the Mail folder just grows and grows, so you can monitor this folder to see if that’s the case.
~/Messages, (If you use the Messages app, they are stored here). If you don’t need all the Messages on your phone etc. to also be on your Mac, you can delete the contents of this folder. (Make sure if you’re using iCloud for Messages that you disable it on your Mac first so your other devices aren’t affected).
Empty the Trash after each deletion to see the results, although sometimes the Mac needs to be restarted to empty the trash at all, or to see the results.
Otherwise, if you can account for all the data, sometimes keeping data stored on an external hard drive is the best way to go. They are fairly cheap especially ones that are large in size. You can store data like your Photos Library and even iTunes Music Library on the external drive, make that the default, and delete the original Libraries, which may come in handy.
This is also a good resource:
https://discussions.apple.com/thread/6651906
