Exporting Apple Mail folders

This question has been raised before but the solution isn't working for me. I just want to put my entire mailbox into folders (inbox and sent) and move them to a hard drive. But when I follow the steps outlined below, the folder that Mail saves is only 12mb. I have 20,000 emails! The folder should be around 1GB with all the attachments. What am I doing wrong? Here are the steps I followed:

1. In the Mail app  on your Mac, select one or more mailboxes, then choose Mailbox > Export Mailbox.
2. Choose a folder or create a new folder, then click Choose.
3. Mail exports the mailboxes as .mbox packages. If you previously exported a mailbox, Mail doesn’t overwrite the existing .mbox file; it creates a new .mbox file, such as My Mailbox 3.mbox."