The YouTube desktop page was designed for 16: 9 video, with videos in any aspect ratio – usually 4: 3 for old TV or 9:16 for vertical smartphone video padded with black bars to suit 16: 9 box. In a change that Google currently rolls out, these black bars are now deleted, and the video box will instead address the aspect ratio of the content displayed.
For 4: 3 content, this means a larger image. It will be both broader and higher, tap the title, description and comment further down the page. For vertical video, the benefits are less visible. Empty space will open up on each side of the video, but while removing the black lines, slim browser windows are less likely to dimension the video. Even 16: 9 video gets a bit bigger due to reduced margins and white spaces.
You can expect a change that makes the videos a little bigger to be taken by YouTube users, but the YouTube Help Forum thread describing the new design and the reason behind it is filled with unhappy YouTube users who want a return to the more consistent design and positioning offered by the old design.
In a change that should be more popular, Google also rolls out a dark theme for the Android YouTube app. The update is still triggering – my phone is not presenting the option yet – but you can see some photos of the feature on 9to5Google. With this update, the Android app is launching the iOS app and YouTube site, both of which have had a dark option for a while. For late night view, the darker appearance is certainly an improvement.