This is a very great extension but it plays as a small cornered window cos your iframe code is as following
iframe
class="youtube_frame"
src="YouTubeUrlHere"
allowfullscreen=""
frameborder="0">

you can easily fix this and make it fill the box by editing the code this way
iframe
class="youtube_frame"
src="YouTubeUrlHere"
allowfullscreen=""
width="476"
height="267"
frameborder="0">