Using Classic Outlook on Windows for personal email, calendar, and contact management
I had the same frustrating experience as most, especially when the MS Office experts kept telling everyone to go set the Text size when the Text size is already at 100% and our issue was not to make it larger. Not sure if this will work for you but after trying to look for other solutions and not finding one, I decided to see what would happen if I change the text size larger. Interestingly, as I set it to 105%, the text size of the ribbons and panels changed (reduced) to the size that I was intended to see. I then changed the text size back to 100% and it now shows the correct size that I wanted to see. Give this a try if you are still struggling.
To change the display font size, select Start > Settings > Accessibility > Text size.
You will see that it is already at 100%. Slide it to 105% and click apply. Then slide it back to 100% and apply again.
Yes, this worked for me also