It depends on the theme in which a video was created. There are two options -- 4:3 and 16:9. Older themes were built with a 4:3 ratio, while newer themes use a 16:9 ratio.
While there is no direct way to change the aspect ratio of a 4:3 video to one using the 16:9 ratio, click here for a creative work-around.
16:9 Themes
- Business Friendly
- Whiteboard Animation
- Video Infographics
- Common Craft
All future themes will be created using a 16:9 aspect ratio.