I mean that you have borders both up-down and left right so you can increase the overall image size by not changing it’s aspect ratio till you run out of the available height or width. (I really struggle sometimes to explain something simple in English, sorry for that:schwitz
To deal with that you should change the aspect ratio to 4:3 (cause that is the A.R. of youtubes video frame) just for the youtube version by adding the appropriate borders so you don’t distort the image.
eg If your video resolution is 1280*1024 to convert it to 4:3 without cropping you have to fit it in a 1366*1024 frame by adding 2 black borders right-left 43pixels on each side. Of course there is no reason uploading it on youtube in that resolution so you could do it in a lower resolution with the same aspect ratio (4:3) e.g 640*480
But right now I don’t see that kind of borders in your video. It’s more like they are equal in height and width.
Anyway it not worth mentioning… cause youtubes quality is already cr@p and the resolution is so small (320*240) that the distortion between 4:3 and 5:4 ar, is not even noticeable.