In the absence of additional information, I would guess it is a memory leak in the Outlook application.
For those who don’t already know, a memory leak happens when an application allocates memory and later loses track of it, so it can’t use the memory and it can’t release it back to the system. When this happens, the only way to recover the memory is to quit the app, at which point macOS will reclaim everything the app was using.
As for what you can do about it, the first step I would suggest is to run Microsoft Auto update (should be on the Help menu of any Office app) to see if Microsoft has released any updates to Outlook, which might include a fix for the memory leak. The next step would be to check to see if Apple has released any macOS update, in case the memory leak is coming from a system framework.
If there are no updates, or if the updates don’t fix the problem, my next step would be to submit bug reports both to Apple and Microsoft. Be sure to include as much detailed information as possible about the bug, including information about the app and platform (hardware model/configuration, OS version and build number, Outlook version and build number, etc.)