What you are seeing on your mobile device is the Splash Page.
The Splash Page is a placeholder for the gallery which is displayed by default on small screen devices when the gallery is embedded in a page alongside other content (rather than displayed on a page of its own with dimensions of 100% x 100%, filling the browser window).
When the user taps the Splash Page, the gallery is expanded to fill the user's browser window.
For more information about the Splash Page and how Juicebox adapts to different devices and screen sizes, please see here.
You can choose to not use the Splash Page by setting showSplashPage="NEVER".
Alternatively, you can force the gallery to be displayed in Large Screen Mode (which, by default, does not use the Splash Page) on all devices and in all browsers by setting screenMode="LARGE".
If you choose to continue to use the Splash Page, you can customize it using the Splash Page configuration options.
When using the Juicebox module for Drupal, Pro configuration options can be entered in the 'Juicebox Library - Pro / Manual Config' section of your Drupal Dashboard ('Home -> Administration -> Structure -> Content types -> Article -> Manage Display -> Juicebox settings'). For example:
Please note that you have also set your gallery's width to be a fixed value of 900px. The gallery will have this fixed width in all browsers and on all devices.
If your Drupal theme is responsive, then you might like to set your gallery's width to be 100% instead.
The gallery's width should then span the width of its parent container no matter what its size.