What Determines if a Software Program is Heavey on System Resources?

Let's say you need to install a firewall, antivirus protection, or a spyware blocker. We hear of programs that are "light" in system resource demands while others make "heavey" demands.

My question is, what are the parameters that determine whether or not a software is light or heavey? In other words, I can understand if a software utilizes 15megs of RAM while running as being light. But where does a software program such as a firewall begin to be labled as heavey in resource demand, i.e., system hog?

Windows Task Manager-

Cnrl -Alt-Del - look at the mem usage before and after you start the app in question.