Youtube video frame size has accidentally got larger. How to reduce it?

I recently hit something in my keyboard while i was distracted and my youtube video frame size suddenly increased. It is not the zoom or ctrl+/- thing. That is still at 100%. It is something else that has changed, and it shows a larger video size only when i open youtube in incognito window. In the normal window, Youtube video frame is still the normal size. And it is not the theater mode either. I have it on default view. Below is how it normally looks vs how it is looking now

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how it normally looks
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How it is looking now

Can anyone suggest how to reduce it back to its normal size?