Gunhead Probably with (AI) Alpha masking. No promises though. But Alpha channel is by default not supported in mp4, so a 2nd black&white data stream would be needed, increasing the filesize.
Masking you can compare to adding a greenscreen background to the image, without altering the original video file.
While if we could use alpha channel, you could make the video file with only the girl, and an 'invisible' background. Which would reduce the file size.
This is Alpha channel (on the right)
This is masking
