Because your gallery is embedded in a web page alongside other content, Juicebox displays a Splash Page in Small Screen Mode as a placeholder where the gallery would otherwise be in Large Screen Mode. When the user clicks the Splash Page, the gallery opens up in a new tab.
When I view your gallery on my iPod Touch 4, the Splash Page is the correct width (displaying the first image in teh gallery as a background image) and when I click on the Splash Page, the gallery opens up in a new tab, filling the available space correctly.
If you do not wish to use a Splash Page, set showSplashPage="NEVER" in your gallery's XML file.
You could change other aspects of the Splash Page via the Splash Page Options.
Alternatively, if you wanted to force the gallery to be displayed in Large Screen Mode on all devices and in all browsers, you could set screenMode="LARGE".
For more information about Screen Modes, please see here.
As an aside, your 'wrapper' <div> does not seem to be closed on your page.
There are 57 opening <div> tags on your page but only 56 closing </div> tags.