(Keep in mind that this does not cover usage of the Hype plugin, but the same rules apply)
What’s happening is that your theme’s breakpoints are determining the available width that your Hype document can use. Your Hype document is showing a layout based on the width available to it.
Some Wordpress themes allow you to display a post without the sidebar; this would provide more width to your Hype document and would show the desktop layout in more cases.