This is a Marketo issue - we've had the same concern. Marketo told us they would have responsive templates in 2015, but that's just an estimate.
We have similar problems with our email templates - small computers don't show the whole email, and some don't even allow you to scroll to see it. We've opted not to use our email templates for now, as a result, and we've made our landing pages to look good vertically, so they would read well even on a phone.
If you google responsive templates and marketo, you'll see some work-arounds for making your existing templates responsive, but I have no idea how well they work.