The email templates will assume whatever the formatting is inside of the Snippet Builder. Some of these formatting issues could arise due to the fact that you copied or cloned some of the example or prebuilt snippets we had in the system, and those may have different formats than what you have inside your template(s).
In order to fix this, you would want to make sure your default font and size under your system global settings > preferences are set to how you want them for your snippets and create new rather than cloning the prebuilt templates we have in there.
If it looks off, you can simply open up the formatter on the bottom of each snippet entry (after selecting the content in the snippet) and reselect the font and size you would like to have applied to the respective snippet.