When you will use the Code directly from YouTube, you are assigning the Width and Height. By default, the iframe Parameters overwrite others. You can remove the Width and Height from the Code or Simply use the YouTube URL Directly, Theme will do the rest
<iframe src="https://www.youtube.com/embed/rH5zO-EL5Fg" frameborder="0" allowfullscreen></iframe>
This is better to Paste the YouTube URL directly in the Video News Box or Post/News Editor in Text Mode. WordPress and Theme will convert the Video Automatically
https://www.youtube.com/watch?v=rH5zO-EL5Fg