In my case I uploaded the 720 x 404 video to youtube, and then the video shown to us get scretched. (It looked like a 4:3 ratio video)
To do point : The Solution
you can do this procedure from here.
Change Pillarbox YouTube Video to 16:9
You may have shared many normal aspect ratio video before YouTube changes it player to larger size. If so, you could edit video info and add yt:stretch=16:9 to the Tag textbox. Another useful YouTube video tricks are yt:stretch=4:3, yt:crop=4:3, yt:crop=16:9. Use yt:crop=16:9 to crop out letterbox and pillarbox bars. Stretch YouTube video to 16:9 widescreen video now to full fill YouTube video player. But note that your video may be short, squat and a bit of chubby looking.
Save the changes, and Done!