The width of the embed is 373px when displaying on iPhone 6 (Max width for iPhone 6 is 375px). The width is 318px when displaying on iPhone 5, however I am mostly concerned with the iPhone 6 width.
When I edit the CSS with Chrome Dev tools, the issue appears to be occurring with the <div class="player-volume"> is interfering with the <div class="player-buttons-right">.
If the volume bar can be scaled down a bit at smaller resolutions, it would be fine.
The other option would be an ability to hide the Twitch Logo on small resolutions with the iFrame. I understand Twitch wants to be branded but it is causing issues for mobile users.