This article describes how to alter your "cached Exchange mode" settings in Outlook i.e. change the setting that determines how much Outlook data is stored on your PC.
NB there is no cached Exchange mode setting for users of Outlook on macOS.
This is a setting within Outlook which determines how many days' / weeks' / months' worth of data (mainly emails and calendar events) are stored on your PC for faster, off-line access. The more data you store locally, the faster the Outlook search function may operate, as it doesn't have to refer the search to on-line servers. However, if you send / receive a lot of emails, have numerous calendar events and / or have Outlook items containing multiple or large attachments, this may have a significant impact on hard drive space on your computer. Cached Exchange mode offers users the ability to set the right balance between speed / convenience and hard drive space used.
Once this process is completed, the size of your Outlook storage file will change accordingly over the course of several minutes, dependent on how much data has to be trimmed or downloaded, and the speed of your Internet connection.