If you have a few sites running on your server, and want to know which worker process/application pool is taking up the most memory. There is a very simple script you can run.
1.) Make sure you have the PID column showing in your processes tab.
2.) Open the cmd window and type the following to get you into the 'inetsrv' folder
and then type the following
appcmd list wp
This will give you a list of all the worker processes with their PID's and show you which AppPool it relates to.