The YouTube desktop site was designed for 16:9 video, with videos in any other aspect ratio—most commonly 4:3 for old TV material or 9:16 for vertical smartphone video—padded with black bars until they fit the 16:9 box. In a change that Google is currently rolling out, those black bars are now being scrapped, and the video box will instead take on the aspect ratio of the content it’s showing.
For 4:3 content, this means a bigger picture. It will be both wider and taller, pushing the title, description, and comments further down the page. For vertical video, the benefits are less apparent—empty space will open up on each side of the video—but even there, removal of the black bars means that narrow browser windows are less likely to cause the video to be sized down. Even 16:9 video is getting a little larger due to reduced margins and white space.