This hint wasn't found by me; I'm just sharing the wealth. Here's the original (in Swedish).
If you imported your 10.3 mail into 10.4, you've probably got some extra files you can delete to recover drive space. Apple describes the process in this tech note; it notes which files can be safely deleted.
Here's a more automated way to delete the files. Open the Terminal and use the following commands (after making a backup; I zipped my mailboxes to another location first):
find . -path "*.imapmbox/Cached*" -exec rm -f '{}' ';'
find . -path "*.mbox/mbox" -exec rm -fr '{}' ';'
find . -path "*.*mbox/content_index" -exec rm -fr '{}' ';'
find . -path "*.mbox/table_of_contents" -exec rm -fr '{}' ';'
find . -path "*.mbox/Info.plist" -exec rm -rfv '{}' ';'
find . -path "*.mbox/mbox.SKindex" -exec rm -rfv '{}' ';'
When done, you are likely to regain a lot of space. My mailbox disk usage shrunk from 620 MBs to 270 MBs.
[robg adds: I haven't tested this one, but if you're going to do so, heed the backup advice. I also noticed that the above version removes cache files, which are not referenced in Apple's hint. This shouldn't cause any issues, since they'll just be recreated, but it may slow Mail down a bit as it recreates the cache files.]

