How is the Compressed Mem column in Activity Monitor calculated?
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.