Chrome: Smart Video Enlarger is an extension that adds two great viewing features to the YouTube web interface.
The first is it allows you to set YouTube videos to automatically take up the full frame on load. This makes the video take up your entire browser window by default, similar to what it looks like when you pop-out the video in a new window. The second is that if you scroll down the page, it will automatically resize the video on the fly, down to a minimum (20% by default) so that you can continue watching the video while scrolling down to read comments.
Even though YouTube has made some changes to their video watching experience over the years, you're still basically stuck with two options: the default video size or fullscreen. Depending on how you watch and navigate videos, this setup is really not optimal nor conducive to your video watching habits. This extension's two features work together to make it easier to get a ‘full' video experience in your browser without having to make extra clicks or going fullscreen.