Near bezel-less displays on Android phones have become the trend in 2017, but if there is one issue, it's that certain aspects aren't quite fully supported yet. A good example of this problem is YouTube and how it doesn't natively punch into the video to fill the screen based on the specifications of your display. While it would be nice if this was a native feature of the app, instead you are at the mercy of the manufacturer and its own customizations.
Google also seems to be applying its own customization for its Pixel 2 XL, as there is a new mode in the YouTube app for the phone. With the new update, you can now pinch-to-zoom on the video that is playing and have it fill up the entire real estate of the display. If you want to adjust it, you are free to do so by zooming in or out. This should be a welcome feature for those that don't want to experience videos cropped on both the left and right edges. However, by using the zooming capability, you do instead end up cropping the top and bottom of the video.
While this could arrive on other handsets sometime in a future YouTube update, it is currently exclusive to the Pixel 2 XL. If you want to try this for yourself, be sure to get the latest version of YouTube, which is version 12.40.