Happy new year! 
I was curious why it seems that the framerate drops as the window/canvas becomes larger
In Case 1 and Case 3, roughly the same scene is shown, so the large difference in FPS is unexpected
Case 2 shows roughly half the scene as Case 1 and Case 3, so the difference in FPS is maybe reasonable
I would be grateful if anyone could share insights on why this might be. Thank you!
In any render engine I am aware of, the number of pixels has a major impact on rendering times. You can try any render intensive game, that is why 8k is much slower than 4k, than 2k pixels and so on.
You can even just get the same effect in Blender, if you play an animation and show the fps (and unlocked the fps from its standard 24 fps 
).
Half the size leads to four times less pixels to be calculated, if I am not mistaken. So I find those values you show not surprising
this is simply due to the gpu limit being reached. Think about the amount of pixels to render and how insane it is when the screen size doubled
