did you look at /var/log?
and also the subdirs of /var/log?
that's the place where all (most) logfiles are stored.
first, you can delete all files that are older than one day (maybe with a cron-job)
second, have a look at the size of the files. if they grow up to many megabyte, anything goes wrong with your system.
third, disable the logging of several services. a good candidate is the firewall. have a search in the forum.
cheers klaus