Pretty much all JSF tools store their entire ResourceLoader in the ToolSession attributes under "msgs". This bloats the size of the sessions.
This seems to be caused by the following:
<jsp:useBean id="msgs" class="org.sakaiproject.util.ResourceLoader" scope="session">
<description>Dynamic Resource Bundle Loader</description>
Not sure how safe it is to take this out of session state but no other tools cache the ResourceLoaders so it is unlikely that it is needed.
Probably the fix would be to change it to:
scope="request" (or omit this)