Using Classic Outlook on Windows for personal email, calendar, and contact management
In Classic Outlook for Windows, you can reduce the Message List font separately.
Go to View > View Settings > Other Settings > Row Font, choose a smaller font size, then click OK three times. “Row Font” controls the sender, subject, and message preview in the message list. Microsoft’s instructions are here: https://support.microsoft.com/en-us/outlook/create-change-or-customize-a-view-in-outlook
If you want the entire Outlook interface smaller, check Settings > System > Display > Scale in Windows and reduce the scaling if it is currently above 100%.