Can't say about GFv3, but even GFv2 loads its stuff lazily. So if you don't use it, it's not loaded.
I've had GF running servlets with a heap as small as 25M I think.
Simply, just deploy Glassfish and your application, and start hammering on the Xmx parameter until you stop getting OOM exceptions during testing.
I will say that I wasn't able to get GFv2 down as small as Tomcat, much less Jetty.
Also, I would avoid the Admin console, it's kind of heavy, but, again, you don't have to "undeploy" it, just don't use it.
I believe that GFv3 has a smaller footprint, particularly for just servlets.
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