Lead Developer
Hi Garth,
The widget of these oEmbed items in WordPress are calculated with PHP (WordPress requires the $content_width variable to be set), which doesn’t know the different between mobile and desktop (unfortunately).
To make your iframe videos responsive, you’ll have to look at an article like this: https://css-tricks.com/NetMag/FluidWidthVideo/Article-FluidWidthVideo.php
Basically, you need to wrap your embedded video in a container with the CSS provided on that page.
Let me know if you have any questions 🙂