The pc version of Outlook has a build in limit of 50gb of stored mail. Even if your mailbox has less than 50gb but you also have a shared mailbox, this gets added to the total.

To increase the size to it's maximum limit of 100gb

Close all office programs
unzip the registry file linked below
Right click the extracted file and run as administrator.
Reboot the machine and Outlook should be set to the max 100gb of data.

https://wiki.vostel.co.uk/assets/dload/vostel/Outlook-365-ost-Size.reg.zip