Please follow the below guide if you think you need (or have been told to) clear down the size of your mailbox.
Method 1: Disable Shared Folder Synchronization
Shared Folders can also quickly increase the OST file size and cause Outlook performance issues. To resolve this, disable Shared Folders synchronization by following these steps:
Click File > Account Settings > Account Settings.
Under the Data Files tab, select the OST file.
Click Settings > Advanced > Outlook Data File Settings…
Uncheck the Download Shared Folders option and click OK.
Restart Outlook and wait for synchronization to finish.
After you make this change, none of the shared folders in the secondary mailbox will be available to you in Outlook when you are working offline. However, all folders in the primary mailbox will still be available to you when you are working offline.