Outlook Archive versus Google All

Is that how many messages you have in your All Mail folder? Do any of the messages in your All Mail folder have labels? If you do not have a folder named Archive, the All Mail folder is synced to Archive - but only messages that don't have labels.

The Archive folder - with the file drawer icon - is used with the backspace key to file mail with one click in a folder that is not named Deleted Items. (Yes, people did that.)

If you use zero inbox with gmail and Archive mail in Gmail (which removes all labels) - it will sync to the Archive folder in new outlook. Messages with labels won't sync - this would create duplicates and increase the storage size in the Microsoft cloud.

If you use classic Outlook and set an Archive folder, that should be used for the Archive feature (but I need to test it more with new Outlook, especially with a folder not named Archive and after the unlabeled all mail messages already synced.)