How is the Compressed Mem column in Activity Monitor calculated? Say we have something like

Process Name   Memory    Compressed Mem
Firefox        2.60 GB   1 GB


What do the figures mean?

Is it:


1GB is compressed and we don't know how much it's compressed by so the app is using somewhere between 1.6 and 2.6 GB but we don't know how much
1GB is the amount of memory saved by compression - ie the app is using 2.6 - 1 = 1.6 GB of actual memory
1GB is the actual memory being used - ie after taking into account the compression of parts of the 2.6GB, we've been able to save 1.6GB and the process looks like it's only using 1GB
Something else?


I intuitively thought it was (2) but a friend thought it might be (3). Now I'm thinking it's actually (1)!
Through various tests on my machine, I have discovered that: The app is using 2.6 GB in addition to the 1GB of compressed memory.

On the Safari Webpage Content processes, if you max out the memory used on your machine, you will see the memory go down as compressed go up. This is how I tested it, and I assume it will work for other apps, but safari usually uses the most memory for me.