Kat,
Good question. The theme has a script built in that will resize videos based on screen size if they are served from youtube.com or vimeo.com.
Although the video is from YouTube, it’s actually embedded from a domain called /www.youtube-nocookie.com
If you can, go back to YouTube and create a new embed code for the video that uses the regular old youtube.com domain and should be good to go.
I’m steadily changing my themes to use a different method for responsive videos so in the future it won’t matter, but for the time being with Moonrise, that’s the best option.
Let me know if I can help,
Bill