How to reduce ribbon size in Outlook
The Ribbon display in Touch Mode:
The Ribbon display in Mouse Mode (the habitual look of the previous versions of Microsoft Outlook):
To change the Ribbon mode, click on the dropdown arrow on the right side of the Quick Access Toolbar and choose the Touch/Mouse Mode command:
The Touch/Mouse Mode button was added to your Quick Access Toolbar menu (see more Adding Commands to the Quick Access Toolbar):
By choosing a specific option, you can change the Ribbon mode according to your needs.
Note: This option works for all Microsoft Office applications at once. If you change the mode in Outlook, it changes in Microsoft Word, Excel and PowerPoint.