Why do I have 8.3 GB mail on my hard drive even when using gmail?
This is a feature of the default set up of Mail.app on OS X. It is storing all your messages for "offline viewing" you can turn this off.


Go to Mail > Preferences click on Accounts
Click on your Gmail account on the left
Then click on the Advanced tab.
Under "Keep copies of messages for offline viewing" set that drop-down to "Don't keep copies of any messages" '


Also make sure that you gmail e-mail account is setup as IMAP and not POP3 you can see the type under the "Account Information " tab.