Storyline output not completely fullscreen on mobile

I've developed a new Storyline project meant for mobile, and meant to be completely fullscreen. No (mobile) player or buttons required.

However, on mobile the output still shows black bars left, right and under the output. See: http://www.starklearning.nl/elearning/Stork/IVO/story.html

How can I disable these bars?

I've already disabled all player features that I could find, and made sure none of the slides use the standard forward/next/submit buttons.