Reading plain text emails
Plain text email is just words and spaces, without any formatting instructions. That is, your email does not specify what font the words will be in, nor the size of the letters. It does not specify the color of the letters, nor the style, like italics or bold.
That is controlled by your subscriber's email reader. Which is good for them. Because they can make the letters in their email as big or small as they want. They can have all their emails appear in the font that is most readable by them.
They control this from the settings in their email reader.
HTML email is different, in that the message contains formatting codes to specify the type of font, the size of the font, etc. So you typically see the email the way the sender wanted. Which for most people, and most of the time, works out fine.