After doing a limited A/B testing of the new look, YouTube has begun a full scale roll out of a new full screen width thumbnails look to the stream site. Instead of having smaller thumbnails with the titles of the just below them, you have have a widescreen looking thumbnail with the title under it in a cleaner font.
The update appears to be a cloud-side update so as long as you are running the latest version of the app, you should see this new look roll out to you over the course of the next few hours.
Functionally, there is no change in the app. This is purely a cosmetic update. For clarity, here is what YouTube looked like on Android before:
And here is what it looks like today after this “full bleed” update:
As I like to say on these types of smaller updates, it isn’t an orbit changer. But it does generally fit better with the redesign efforts that have been going on through the suite of Google apps.