My video player is resizing by itself! What gives?

The problem is likely that your theme's CSS (Cascading Style Sheet) is managing the <video> tag. For instance, the WordPress Twenty Twelve theme has an entry which looks like:

	/* Make sure videos and embeds fit their containers */
embed,
iframe,
object,
video {max-width: 100%;}

Commenting this element out or removing it from the cascading style sheet will resolve the issue.