JBoss can be as "light" as you want it to. I think the minimum configuration is like 80K and hardly uses any memory -- though it has no userful services running on it.
The first thing
you should do with JBoss is figure out what you don't need and customize the server configuration to have only the services that you'll use. For example, if you don't need JMS, strip out the messing service. If you don't need web services, remove them. You get the point.
Peter Johnson and I cover this fairly well in JBoss in Action (which I'm plugging since we're doing a promotion here this week). We have a section in Chapter 15 on "Removing unwanted services".