If you set the Device Viewzoom to Page Width or Device Width, it should match the width of the iPhone:
It appears in this particular case that given the page width and the width of an iPhone in portrait mode, the scale needed for the width of the page is smaller than the default setting allowed for the device. This can be fixed by adding a minimum-scale value to the viewport tag:
<meta name="viewport" content="width=1536.00,minimum-scale=.2">
This extra code will automatically be added in a future version of in5 (1.6.2 or higher) when either Scale to Device Width or Scale to Page Width is selected as the Viewport Zoom Option.