One of the apps in my Containers folder is taking up a lot of space.

Hello,

I suggest you contact the developer of that app.

You were right to avoid deleting the container. Although they aren’t “system” files, they are “special”. The app will get confused if you make any changes to it. Some of this data is also cached by some system processes, which contributes to confusion.

If you definitely don’t want the app and do want your space back, you can delete that container manually or with Omni Disk Sweeper. Make sure to immediately restart afterwards. That should minimize the damage. The container might come back due to the system processes that are involved. But hopefully you will get those 15 GB back.

Also, there is a possibility that Time Machine APFS local snapshots will hold on to that data. You may need to delete them. The system will take care of this storage and return it eventually. But if you are paying attention and looking for it, you might notice some funky behaviour.

Did you read the last article I posted a link to? It contains instructions on how to delete local snapshots.

Did you read the last article I posted a link to? It contains instructions on how to delete local snapshots.

That might help. Time Machine did have local snapshots before APFS. They weren’t the same back then, but it won’t hurt to try.

Disk Space - Other      in High Sierra and later, this is now included in System

Disk Space - "Other" and What Can I Do About It?

Local Snapshots on Portable Macs

So you're saying if I do delete it and then ever want the Line app reinstalled then it will just rebuild those containers supposedly?

No. I’m saying the exact opposite. Only delete that container if you are sure you never want to use the app again. Otherwise, contact the developer.

There are a number of “other” types of data that are lumped into the “System” category. At 57 GB, you are actually doing pretty good. Here is what mine says:

And even this is 25 GB better than when I checked it 30 seconds ago.

One thing to watch out for is that Apple prefers to use the word “available” rather than “free”. Free space is always the hard limit. That is what the system checks for. But the only thing it is going to tell you about is “available”, which is a hypothetical value that may or may not become useable at some unspecified point in the future depending on a number of different conditions.

It is a relatively simple thing to do, so I thought it might be worth trying.

Sorry. I assumed you were using Mojave. You don’t have APFS in Sierra. This is not an issue.

Have you emptied the trash?

There are some bugs with those system space estimates. I’m not sure if those bugs existed in Sierra too. The amount reported by “System”, and your free space, may simply be incorrect. Since you don’t have APFS or local snapshots to complicate things, you can trust Omni DiskSweeper.

Otherwise, everything I said about containers is still true. With 120 GB, you are just going to have to be brutal about what data you keep on the machine.

That display of storage isn't the most accurate. Select your hard drive in the Finder Sidebar and do a Get Info (command - I). How much free space shows there?