Update: Aaaaaand its back (1/26/2015).

Gmail on iOS started to bump up font sizes on some emails by 50% a few days back and for a lot of emails it actually improved readability. But if your email is one of the unlucky ones you can use a quick fix to prevent Gmail from increasing the fonts on your email (for now):

<div style="display:none; white-space:nowrap; font:15px courier; color:#ffffff;">
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</div>

More information here: Gmail iOS App Increases Email Font Size and What You Can Do About It

On the Campaign Monitor blog Vlad also noticed you could prevent Gmail from increasing the font size by adding !important after your font size declaration.