Classic Email Template text auto-resizing

In Salesforce Sales Cloud, my org is using Classic Email Templates. In conjunction with email templates that we have set up, we use a Classic Letterhead. After a sale one of our classic email templates sends. 

When a contact receives that email, some text portions of the body paragraphs are appearing in a different size. I have tried going into the HTML editor to highlight, select all, resize and save but that doesn't seem to be doing the trick. 

Any ideas?

#Service Cloud