so I would like to add my couple of cents to the question.
I have the same problem, and I think it is related to a bug in the Outlook App for Android, when you use connected IMAP accounts.
In a previous post one gentleman listed the number of files created inside the outlook app folder.
I assume he had a rooted phone or set the app somehow to debug as access to this folder is not possible without root permissions.
I for myself had the problem that the app consumed around 30 - 40 GB with 3 linked accounts, 1 IMAP, 2 Exchange.
In my case (Samsung Galaxy S9 later S10) an error message came up that there are too many files and the phone need to be reset. Obviously after that error message creating new files (e.g. taking pictures) was no longer possible, causing several apps to crash.
I suspected another app creating random files, but thanks to this thread I realized it was the outlook app. I have now removed the IMAP account and the size has decreased to less than 1 GB.
In my case it was a (German) GMX account. The outlook app as had issued with this account in the past (e.g. adding it was only possible by using gmx.net instead of .de and then change the authentication).
I don't think there is any way for an end user to fix it. We have to hope that somebody @microsoft is reading this and then realizes he needs to fix something.
So if you need to use IMAP you are better off with any other mailing app for now