[exim] callout db quite large?

Hello all,

Running a FreeBSD box w/ exim on it as a filtering mailhub.

sender callouts and recipient callouts are configured and working as they should.

`which exim` -bP | grep callout
callout_domain_negative_expire = 1h
callout_domain_positive_expire = 1h
callout_negative_expire = 1h
callout_positive_expire = 1h

ls -l `which exim`
lrwxr-xr-x 1 root wheel 11 Dec 15 12:59 /usr/local/sbin/exim -> exim-4.60-0

My question is.. should the callout db be so big?

du -sh /var/spool/exim/db/callout*
  78M    /var/spool/exim/db/callout
   0B    /var/spool/exim/db/callout.lockfile

exim_tidydb -t 20m /var/spool/exim callout > /dev/null

du -sh /var/spool/exim/db/callout*
  78M    /var/spool/exim/db/callout
   0B    /var/spool/exim/db/callout.lockfile

it just seems to never get smaller.. am I missing something?

I have no 'reason' to make it smaller, I just wonder why it's so big.. the other files aren't
anywhere near as big.

[/var/spool/exim/db]# 81 > du -sh *
  78M    callout
   0B    callout.lockfile
  96K    retry
   0B    retry.lockfile
160K    wait-bsd_smtp
   0B    wait-bsd_smtp.lockfile
  64K    wait-greg_smtp
   0B    wait-greg_smtp.lockfile
  64K    wait-remote_smtp
   0B    wait-remote_smtp.lockfile

Thanks in advance.