To keep down the size of your local mail, you need to compact them periodically. At least with Eudora and Outlook, new messages get appended to the end of the file, while empty space for messages which are deleted or moved are not overwritten. Compacting the mailbox eliminates those gaps.

Perhaps Mozilla and/or Thunderbird do this automatically, and I don't doubt they may have a more efficient compression algorithm, but it's also possible that the dramatic savings comes from the import process simply stripping out the offending empty gaps.

As far as size of Outlook PST files, 1GB used to be the upper limit, but I thought it had been expanded with Outlook 2003. I archive every six months or so and none of the files is over 1GB, but I only have about three open at any given time.