Why is youtubes video player suddenly giant?

This suddenly happened last night, the video player on youtube is now almost twice as big, it looks awful on my small laptop screen and eats up a lot of cpu when playing. I can't change it back, and can't find anyone with a similar issue. I've tried deleting cookies, making sure i'm not zoomed in on the page, running firefox in safe mode, nothing.
I've contacted both firefox and youtube support, but haven't gotten any answers. Attatching pics of what it looked like before and what it's doing now

Not sure if it's just a weird coincidence or what, but it also happened right after avast updated? not sure how that would factor in.

OK do you see the rectangle next to the full screen button on the bottom right try pressing that it might work

That was the first thing I tried

have you just dragged the window a bit bigger? youtube's interface will resize depending on how big your browser is - maybe it's this?

also, i have a chrome plugin called "ImproveTube" which lets you do things like control the size of the player - it has a "large" mode which makes it look similar to what you're reporting. maybe someone's installed this on your computer?

My browser's always full screened, never done this before
I THINK this happened because after avast updated, the web security button shrank and now my toolbar is like three pixels smaller, not sure if that was enough to cause this though

My browser's always full screened, never done this before
I THINK this happened because after avast updated, the web security button shrank and now my toolbar is like three pixels smaller, not sure if that was enough to cause this though

If you're still insure what caused your display issue It looks like mine did after I unknowingly activated the Snap Assist feature on Win10 from Desktop mode with one open window.

Not an answer but a clue

Edited by ChuckPasadena, 26 November 2015 - 06:34 AM.