[Evolution] What is the purpose for the files besides mbox?

["Randolph S. Kahle" <RandyKahle KahleAssociates com>]

I am trying to reduce the amount of storage used by my installation of
Evolution.

What is the purpose of the following files?

    -- folder-metadata.xml
    
    -- local-metadata.xml

    -- mbox (this stores the messages)

    -- mbox.ev-summary

    -- mbox.ibex


If I have a folder with a lot of messages and then delete them, I find
that mbox.ibex remains very large (e.g. 18M). How do I compress this
file when / after I delete messages?

 -- Randy