The particualr change about the back bars came down to using the maxresdefault image instead of the hqdefault. Not all videos have a high res thumbnail though, so in those cases, we fallback to the opengraph image, which is equivalent to hqdefault.
That image size has a different aspect ratio and includes black bars. But in both cases, we have the aspect ratio fixed to 16:9, so the black bars should not be visible when displaying the player.
Here’s an example of one video that does not have a high res image and it’s using this one for the thumbnail.
Can you share some examples where this is happening?