Many emails are designed with a large screen in mind. Text that looks great on a large screen can be difficult to read on a mobile device, though. If Gmail deems that the font-size of any text in an email is too small to be legible, it will increase the size and flag the message. That’s pretty nice. What could have been illegible is made legible, eliminating the need to pinch and zoom our screens. It can also be a little frustrating.