Same here, Outlook has been open from 9am > 4pm and it's using 64MB.
Is it minimized? IIRC on Windows, when an application is minimized the working set is trimmed which can result in a significant reduction - but it jumps back up when you start using the application again.
Maybe a Mac. My Outlook on mac is idling at 150MB.
Yes, this is Office 365 on macOS. Neither the computer nor Outlook have been restarted in weeks, which is surely contributing to its current RAM footprint.