Right now, on my 32" 2160p screen, when I either maximize my browser window or put it into "fullscreen" mode, YouTube shows me a centered section with useful information (wide enough to display four videos when visiting the "/videos" endpoint), and empty space to either side of that section that's wide enough to convert this single centered-column layout into a three-column layout... tripling the amount of data on screen.
Both this and whatever "leanback" thing YouTube is testing are both pretty godawful. I do prefer the wasted space, so I know I can rearrange my windows to make use of the space. You never know whether or not a thumb-centric UI will shrink itself down when the viewport's size is reduced.