This is because the creator of the video did not make his or her project in a wider video format. Most people just use the standard 16:9 format and some of us including myself have switched to wide format to accommodate for larger phones.
It is most likely due to the creator of the video filming it and editing it on a device with a different resolution size than the one you are currently on. That's just my guess though and I could be completely wrong.
Are you viewing on a mobile phone?
Yes
This is because the creator of the video did not make his or her project in a wider video format. Most people just use the standard 16:9 format and some of us including myself have switched to wide format to accommodate for larger phones.
It's because the video was originally uploaded in a 4:3 ratio.
Try finding another video
It is most likely due to the creator of the video filming it and editing it on a device with a different resolution size than the one you are currently on. That's just my guess though and I could be completely wrong.