For you Mac OS X 10.3 folks running Mail.app:

You may or may not notice that content_index files get very large in your ~/Library/Mail folder.

To find them do "find . -name "content_index" -print0 | xargs -0 ls -l

If you have many mailboxes and accounts this can get pretty big... I checked mine today and it was 1456KB.

I used this one liner script to calculate:

echo `find . -name "content_index" -print0 | xargs -0 ls -l | awk 'END { for (i = 1; i < NR; ++i) print "+"; print "1024/p"} {print $5}' | dc` kilobytes

It's using awk to pipe commands to dc... reverse polish notation rules.