Thanks @bangank36. I deleted the width and height inside the embed code, and now the video fits better on mobile. It's a small video on desktop, but it's OK since most of my users view my pages via mobile.
For reference, here was my original embed code:
<iframe width="560" height="315" src="https://www.youtube.com/embed/dQw4w9WgXcQ" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
and here is the updated embed code now:
<iframe src="https://www.youtube.com/embed/dQw4w9WgXcQ" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>