CouchDB will keep updated and deleted documents in old revisions. On heavily-written databases this will cause their size to grow up with time.
You can use this script using cron to periodically compact your databases: https://gist.github.com/4483412
Assuming you saved it in
chmod 755 it, you can cron it like this:
@weekly /usr/local/sbin/couchdb-compact.rb >> /var/log/couchdb/compaction.log
to also keep an activity log of the process.
Please keep in mind that you should attempt compaction of heavily-written databases during off-peak hours, or the compaction won't be able to catch-up with the writes. More information here: http://wiki.apache.org/couchdb/Compaction