Mail - Gmail Font

Not sure what happened. I have been using Mac Mail with my Gmail account for over a year now. Then all of a sudden yesterday, when reading emails - the email is too small. When I click on messages on my regular email account, the display is normal. The fonts seem okay, but it looks as though I have to zoom in. But if I zoom in on an individual Gmail mail, the header gets bigger, which tells me that that is not the problem.

This all seems weird. Has anyone run into this problem before?