If youtube is in the new 2017 YouTube Material Redesign, automatically restore classic view
< Feedback on Youtube - Restore Classic
Could you post a screenshot?
Can you try going to new youtube (www.youtube.com/new) and see if the player is small? And go back to classic with the script, still small?
I'm not sure if I'll be able to fix the problem since the player has nothing to do with my script.
Problem with the player
the player is displayed small and try to uninstall it and remains the same