I don't know if this is common knowledge, but I'd never heard of it before.
I noticed that my "Inbox" file for Thunderbird was surprisingly large (100M+) even though there were very few emails in there and most of them were tiny. On a second account the Inbox file was 67M and that was also a bit suprising.

After some googling I came across the "compact" command which can be run on a folder from the right-click menu. After doing that, the file shrank down to 1.3M, which is much more like it. I've no idea why this isn't done automatically by default, but there you go. We do have a defrag tool after all. Might be of use to someone if they've had their mailbox for a whlie, with lots of deleted spam, especially as there's a size limit beyond which Thunderbird starts misbehaving...

More info here: http://kb.mozillazine.org/Compacting_folders

Hmmm.

I use thunderbird 2.0.0.6 to serve up email via IMAP. I just checked my 1.5 year old inbox (which is kept pretty clean) and it was a svelte 74.8KB. I wonder why yours was so large?

Thanks for the heads-up.

I get a boatload of spam on that account. It's the email address of a website, so it gets loads, and over the past year or two a lot of that has been image spam (to try and defeat the text filters).
I always thought that once I'd deleted the spam and then emptied the Trash folder, they were gone. But according to that article, they weren't really deleted.

I had the same, I just used to click File and Compact Folders. It was acting like a database I suppose, in that even though you deleted the message, the empty space was still there and you only got it back in terms of disk space after the compact. Thunderbird uses mbox format, but has some other files as well for speeding things up a bit.

When Thunderbird is setup to use IMAP it does not store mail files on your computer. This keeps your inbox file very slim because your bulky mail files loaded with attachments stay on the server. POP takes your mail from the server and saves a copy on your computer. This would explain the huge differences in the size of the mailbox files.

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IMAP can store your emails for offline use.