Hi Doug,
Not sure if you are aware of this, or if it relates to your question, but perhaps have a read through this YT post on how to select a different embed size when getting the embed code:
https://youtube.googleblog.com/2010/03/new-default-size-for-embedded-videos.html
Basically, when you click the Embed item on Share, you get the default iframe embed code highlighted, ready to copy. Click the Show More link and then you can choose the Video size from that drop down selector (default is 560 x 315).
Cheers!
Lyle