I can watch YouTube videos on my smartphone in half screen (vertical screen orientation) or full screen (horizontal screen orientation).
Or I can use my smartphone as a router and watch the video on the latptop, in small size, large size or full screen:
From a bandwidth consumption perspective, does it matter? When the connection is slow, will watching the video in a smaller size help?
YouTube does automatically change the quality based on the player size by default, which means watching it in full screen will increase bandwidth usage.
If you want to stop YouTube from automatically changing it to a higher quality version, then log in to YouTube, go to Settings → Playback in your profile. Then choose "I have a slow connection. Never play higher-quality video."
Alternatively, if you only have a slow connection occasionally, then you can simply click the change quality button (the cog) and set it to a lower setting.
As long as the quality remains the same, then changing the player size will not effect bandwidth consumption.
