My Mac was down to 7.3 GB of free space last week, so I went looking for what was eating it. The biggest file on the machine was easy to spot: a Docker.raw file with a size of 1 TB.

My SSD is 228 GB.

If you searched "Docker.raw 1TB" because your Mac showed you the same thing, you probably had the same moment. A file four times bigger than the disk it lives on, and your first instinct is to delete it.

Don't. Here's the output that changed my mind:

$ cd ~/Library/Containers/com.docker.docker/Data/vms/0/data $ ls -klsh Docker.raw 1844848 -rw-r--r--@ 1 priyanshutiwari staff 1.0T Oct 30 07:48 Docker.raw

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Two sizes on one line. The 1.0T near the end is what the file claims. The 1844848 at the start is what it actually uses, in kilobytes. That's about 1.8 GB.

So the scariest file on my Mac was innocent. This is what I learned chasing it, and where my space really went.

The one idea: a file has two sizes

Most of us think of a file's size as one number.…