The Auto Archive feature of Outlook is a feature which create a pst file and move automatically items that are older than a number of days you configured into that pst file.
When migrating a user to Exchange online with the online archive enabled, the auto archive to pst feature of Outlook disappear from the Outlook options but the feature is still enabled and you cannot turn it off anymore.
Which means that no matter the user is having an online archive, the pst file will continue to grow…
In order to turn it off, you can do it via a modification in the registry : HKCU\Software\Microsoft\Office\<OutlookVersion>\Outlook\Preferences\DoAging
This value is set to 1 when auto archive is set up. To turn it off, just set it to 0. You can do it via GPO if you want.