One thing to keep in mind is that when you specify the font size in the email editor, you’re choosing the size based on pixels, not points. Pixels and points are different units of measurement for text size. Think of the difference between inches and centimeters - you can have a value of 4, and 4 inches is going to be much longer than 4 centimeters.
Sometimes, when email clients (like Outlook) render messages, they will convert pixels to the closest point size. This means that in some email clients, the message text gets rendered in a different point value than the pixel value you specified.