Maximize multi-stream player sizes in the popped-out player

I often like to watch two streams atop each-other as shown in my screenshot below (slightly larger than half of a 1080p monitor). As you can see, there is plenty of unused black space around the edges which causes both streams to be much smaller than they could be which is pretty frustrating. I understand some dark areas might have to remain depending on the size, but it could at least be a lot more filled than this. The free vertical space may make it seem like not much remains but this extends to much more horizontal space due to the aspect ratio and makes it a lot easier to see what's going on in two streams. An alternative solution/feature could be to individually popout streams, since it's properly maximized with a single stream and I could resize them however I prefer.