The problem you're seeing is because your landing page template has meta viewport setting that is restricting mobile browsers to render the page as if it has 640px width.
<meta name="viewport" content="width=640, user-scalable=no">
For a responsive page, you'd probably want a viewport more like:
<meta name="viewport" content="width=device-width, initial-scale=1.0">
That will set the width of the page to the width of the device, so that it scales based on the device size.
Did you know that Marketo has recently introduced new mobile specific editor and mobile compatibility mode that automatically does this for you? You can get started by making your templates mobile compatible in the template editor. If you go this route, you'll also probabably want to remove any meta viewport and CSS media queries in your templates, and let the Marketo Landing Page editor take care of it.
You can learn more about it here:https://docs.marketo.com/display/DOCS/Add+a+Mobile+View+for+Your+Landing+Pagehttps://docs.marketo.com/display/public/DOCS/Make+an+Existing+Landing+Page+Template+Mobile+Compatibl...