Slider too big on mobile

I’ve installed the Sydney theme here:

And it works great, for the most part.

The header sliders look great on my desktop and laptop, but are too large on my Galaxy S5:

Well, at least initially they’re too large, then after a few minutes, they resize themselves and look fine. That is, I can see the full width of the image and the Call to Action button without scrolling down.

Does anyone have any idea what might be causing this glitch, or how I might be able to fix it?