This is not a bug. YouTube is experimenting with a new player which is resizing the Player Dynamically. Read here: YouTube Experiment
There's a new YouTube experiment that changes the video player's size,
depending on the browser window's size. When resizing Chrome's window,
YouTube switches from 360p to 480p and to 720p, while the player gets
bigger.
Are you sure you deleted all your cookies ? Try this, launch your Chrome Browser in Incognito Mode (Ctrl+Shift+N) and go to YouTube and check if you still see the large player. If not than you did not clear the cookies properly.
Regular YouTube Player
To go back to the regular interface press Ctrl+Shift+J in Chrome and Copy/Paste in the console the following code:
document.cookie="VISITOR_INFO1_LIVE=; path=/;
domain=.youtube.com";window.location.reload();
Large YouTube Player
To set the large YouTube player press Ctrl+Shift+J in Chrome and paste the following code which will change the YouTube cookie:
document.cookie="VISITOR_INFO1_LIVE=E_OLzg3yeLw; path=/;
domain=.youtube.com";window.location.reload();