Can you confirm how exactly you are exporting your Outlook.com account?
Settings -> General -> Privacy and data -> Export mailbox
#1 - It can take up to 4 days for the export process to complete at which point you will be able a PST file which contains the entire <Mailbox>. The larger the mailbox, the longer it takes
It's been weeks if not longer. And actually I just checked it today and it seems to have reset, it no longer shows the export in progress and I did not receive a link to the file.
#2 - Exporting a mailbox does not remove anything from the <Mailbox>
I understand, it is my intention to delete my old emails after downloading the archive.
#3 - Just as aside, the sheer fact that you have an Outlook.com account means that you have a free 5GB OneDrive account (both use the same login credentials).
My inbox currently takes up 15 GB, so unless it compressed below 5gb then that doesn't help me much.