what resolution TV ? 1080P or 4K
the phone might be near 1080P and could be less
also many sites automatically stream a lower RES picture to the phone
could be a small difference or a huge difference, in the amount of data use
probably the best way to get the lowest amount of data use per movie
is to stream them 'ON THE PHONE' then cable to the TV to use the cast/mirror function to send the phone display to the TV via wifi or bluetooth