There are many possible reasons for the the lower power consumption. E.g. the OS could schedule processes other than the foreground window less often in fullscreen mode. Or it even detects that the user watches video and puts the CPU in a very low power (low frequency) mode. Or it's the overhead of the desktop environment and compositor. I have no idea.
I justed wanted to give an explanation why I don't think that the video scaling makes a huge difference.
And that makes perfect sense. These are all hypotheses to be ruled out, at this point though. Indeed, your "schedule processes" one is essentially the other idea I threw at the beginning. My hunch is more likely that is the deal. I'd have to eliminate both through testing, not reasoning, though. Reason can not and will not ever eliminate why something behaves against expectations.
