Configuring preferences and options in Outlook.com
I have exactly the same issue using Outlook under Android 10. As a consequence I've just uninstalled it and will continue to rely on Gmail. Prior to discovering that Outlook was the cause of me running out of space I scrapped my previous phone and bought one with a larger storage capacity. Unfortunately the issue still persists and has been ongoing for several months. Microsoft really needs to get its act together and sort out this issue.
You should sue Microsoft for the cost of your new phone (just joking).
I have to delete my User Storage every 3-5 days and then re-add my IMAP account. I reset yesterday when the User Storage reached16+GB, the phone memory was100% and most apps stopped working. Today Outlook User Storage is just a few hundred MB. It will explode again soon. I keep complaining to MS support. They say it is a known problem but no date for a fix. Don't hold your breath.