Sue, I did not mention that I also set the Placeholder to be Full Width - this balances everything.
It is needed as Xara centres the web page and it is being recalculated each time it is adjusted.

I set mine to 404px height with no problem.

You are playing a native 640px video scaled up.
If you reduce the browser width to just when you jump to the variant, you will see it render at native resolution and all is crispy.

Acorn