Mail using 78GB of space

Macbook Pro early 2011.

According to 'About this Mac' the Mail application is taking up 78GB of space. However I have Mail preferences set to have the email hooked up to it offline and inactive. I do not receive any emails to the Mail application and no emails appear when Mail is open.

Can someone shed some light as to why or more importantly how I can fix this because 78GB for literally nothing is ridiculous.