In looking at why the memory usage is so high in jboss5, a main culprit in heap usage from startup is the vfs. The all config will not start with 128m max and the bulk of this is due to strings referenced by the vfs code. Many are URLs, many others are jar entry related.
The current complete mapping of jar entries to virtual file handlers is going to have to be redesigned to limit this. The associated mc issue is http://jira.jboss.com/jira/browse/JBMICROCONT-156.
