"This assumes that you're using Gmail IMAP. Set Mail.app to not keep offline copies of messages. Mail -> Preferences. Accounts -> Gmail account -> Advanced Tab. Set ""Keep copies of messages for offline viewing:"" to ""Don't keep copies of any messages.""
EDIT As per Steve Folly's comment, it's true that with that method, Spotlight would no longer be able to access those emails for searching within entire messages. If the 11 GB of your email is mostly due to a lot of attachments, another option would be to choose ""All messages, but omit attachments"" in that same dropdown box. That way it would cache the text portion of the mail and you would still be able to search through it."
The Mailbox > Rebuild option worked great on an IMAP connection that stopped showing my older local email. It's a low-impact first thing to try before you start deleting files.
