Whilst there is a responsive column layout now included in the core CSS, there is a missing viewport header to tell mobile devices the page is responsive and to use the device-width (rather than trying to use desktop site width).

I suggest adding the meta "viewport" tag to the HTML head of the page.
<meta name="viewport" content="width=device-width, initial-scale=1" />

Documentation on the use of the viewport tag can be found at the below links:
http://www.w3schools.com/css/css_rwd_viewport.asp
https://developers.google.com/speed/docs/insights/ConfigureViewport