We don't use only PWB. Danny_B made some work recently so that all dumps are in the same place etc., what consumed the most space were duplicates. This was supposed to be documented on https://wiki.toolserver.org/view/User-store but it's never been. Most space is taken by view stats whose compression is discussed here: https://wiki.toolserver.org/view/Talk:User-store
There is some basic doc in "readme" files in /mnt/usr-store[/dumps[/...]] though.
However, thanks for reminding me the wiki doc, I'll try to put it on ASAP.
Danny B.