I have this problem too! Glad to know I'm not the only one who has this problem.
Maximized youtube videos in second screen revert to default size when I am active or more specifically, clicking, in the first screen...a bit of a disappointment! I came across this thread through a web search, hoping to find a solution for this...but no cigar.
Anyone know how to keep a youtube video maximized or locked in full screen on one monitor while being able to interact with the other monitor? Badger94z28, I wait patiently along your side. Someone...please...help...