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.