You may find that videos expand to fill the screen when viewed in mobile Safari on iPhones. In many cases, this is expected behavior.
Apple iOS 9 and Older
Mobile Safari automatically scales videos to fill the screens of iPhones running iOS 9 and older. (See Apple's developer documentation for details.)
If you need to avoid this behavior, we recommend viewing courses with videos on larger screens, such as iPads, laptops, and desktop computers. Or, create a simplified version of the course that doesn't include videos for iPhone users.
Apple iOS 10 and Newer
As of the January 26, 2017 update for Storyline 360 and Studio 360, videos play inline (rather than fullscreen) on iPhones running iOS 10 or later. Just publish your course and it’ll automatically work.
Videos in Storyline 1, Storyline 2, and Studio '13 courses always fill iPhone screens, even if they're running iOS 10 or later.