Because one size does not fit all.
PocketMonitor creates a new video at the top with scaleable video size to maximize the amount of viewing space. in a new tab, the video is opened that is scalable to the browser size.
1. Go to YouTube video page
2. Click the PocketMonitor Icon
3. Full Screen video!
Youtube currently has theater mode alongside full-screen video sizes, but other than that, there is no size for in-between. In many cases people use full-screen alongside multiple monitors, but in some cases if you don't have another monitor, you would need to suffice to one of these three sizes.
I made PocketMonitor because I wanted to have a quick or working way for Youtube videos to be resized based on the web browser size.