All embeds default to 300 pixels width – why ?

After WP 3.6 – all video’s I will embed inside my Posts are created at 300px width ( default )

I don’t wanna use short codes or special plugins, Responsive content should be straight out of the box.
I just want to include my browser-URL of the video ( YouTube – Vimeo ) and let Oembed do everything else but for some reason after 3.6 – WordPress creates 300px width video’s.

I even have this in my functions.php file.

if ( ! isset( $content_width ) ) $content_width = 700;