I have had the same issue but with a 40mb file which is consuming my hard drive. The Apple Mail message in Recovered Messages "On My Mac" in the left hand column has one message in it that keeps duplicating infinitely.
As you know deleting it does nothing. It reappears. I've tried deleting and quitting, changing settings for deleted mail storage, deleting that "mailbox" and the mailbox reappears too. I am rebuilding all mailboxes right now and will let you know.
Side note: The message shows a "Message viewed on" date of Dec 31, 1969. That date used to be associated with a bad clock battery on older macs. Not sure if it is relevant here.
if you're up to using terminal, try this: Make sure Mail is quit, open terminal, cd down into your mail folder for the account that contains that message (should be something like ~/Library/Mail/IMAP-your_email, maybe POP instead of IMAP) and run the command ls -al. If you see a hidden folder called .OfflineCache, delete it with rm -rf .OfflineCache. that may get rid of the message that's constantly being recovered.
The process above worked for me with one extra step.
After deleting the .OfflineCache and before starting mail ...
Navigate to your ~/Library/Mail/Mailboxes/Recovered Messages (for the account in question) and remove the folder and plist file under the appropriate Recovered Messages folder ...
If you can't see the Library folder in Lion, go to your terminal and type: chflags nohidden ~/Library/
*disclaimer - the crazy reappearing recovered message hasn't reappeared in the last 5 minutes - LOL*
Deleting that folder freed up 700 GB for me yesterday... there was some crazy schnizzle going on. Yes 700GB not MB.
Definitely a bug. I have the same issue. File was 33.1 mb and I've released 165 GB by using OmniDiskSweeper. Unfortunately, the recovered file recovered itself and I just found 6 more copies, 1 per second. It's faster than I am. I am not a programmer but will try to follow the advice above as it appears to have worked. Yikes.