Storyline 360 Modern Player - width of controls

I have a custom Slide Size of 1296 x 740 pixels. I am using the Modern Player with the player controls and the side panel is disabled. From the Player setup screen it looks perfect. The controls at the bottom are lined up perfectly with the slide placeholder above. However, when I preview or publish, the controls extend way beyond the edges of the slide. The seekbar is huge! Is there anything I can do to keep the controls lined up with the width of the slide? See attachment.