Our emails display correctly when previewed in Marketo using the desktop and mobile device views. Our emails also display fine when tests are sent. We have some users receiving our emails and the emails are still legible but the message content (text, photos, links, etc) is significantly smaller when opened on a mobile device. Could this be due to the mail client? If so, are we able to fix this on our end?
It's super hard to code a decent looking HTML email that's compatible across all email clients and device types.
You will need to use something more comprehensive than the marketo previews to test your email code, like Litmus, and adjust your code accordingly.
Note the type of device and the client/app that doesn't render how you intended the email to be rendered and trouble shoot accordingly, but beware that a fix for one mail client may cause issues in another.