Phil,
Sorry, I have been looking for information on that popup in the Moodle.org site and haven't been able to find anything. My sandbox isn't letting me do the auto linking this week for some reason and I haven't had a chance to play with it.
The two things I can think of is that there may be a theme problem which is causing this. If your site allows you to or if you are an admin, you might try a couple of different themes in your course to see if that makes a difference in how the popup shows up. The other thing you might try is embedding the video code into the definition by copying the embed code from share>embed on Youtube and placing it in the html code section <> in the text editor. This might work better for your popup. That code has the iframe dimensions in it and you might be able to adjust it a little if it would help. ( One more: make sure you are not centering your video either, left justify it.)
Maybe someone else has had this problem before and can help better than I.
![cool cool](https://moodle.org/theme/image.php/moodleorg/core/1713509870/s/cool)