Why is the size of my .Private folder so large?

Last night I had about 190GB of free space on my 500GB HDD. Today I have about 80GB free. I ran df and discovered that my /home/jon/.Private folder is currently using 80% of my hard drive.

What. The. Hell.

I really don't need to encrypt my files that bad. Can anyone tell me why I've lost so much space to this, and what I can do to recover as much free space as possible?

I realize that I'm not going to get back 330-something odd GB of space, but I lost 100GB overnight. I'm new enough to Ubuntu (and Linux in general) that I don't want to proceed without a firm understanding of what's going on here.

Thanks in advance, guys.

Since its contents are encrypted, you are unlikely to be able to tell much by looking at the files in ~/.Private directly.

Instead, you'd be better off looking at the unencrypted view of those same files in ~/Private. The ecryptfs system has quite a low overhead, so if ~/.Private is large it is likely because you've placed a lot of data in ~/Private (or a program running on your behalf has done so).

I just went through a similar situation on Ubuntu 16.04. Not having ever set up the ~/Private folder, but with the ~/.Private folder taking up 90% of disk space available. In my case I had enabled the /home folder encryption option during install; it seems that in such case the ~/.Private folder is set up automatically for the purpose. You can verify it, if the ~/.Private folder is a link to /home/.encryptfs this is most likely the case:

$ ll ~/.Private
lrwxrwxrwx 1 user user 33 nov  1 16:25 /home/user/.Private -> /home/.ecryptfs/user/.Private/

I do not know exactly what causes this expansion of the encrypted file system, but it is basically the ~/.Trash folder. I just needed to empty the Trash in the DE to bring things back to normal. I suspect that when single files are removed from the Trash they somehow remain in the encrypted file system; emptying the Trash fixes this.

Note that it may take several minutes for the system to report the correct space usage after the Trash is emptied.