Oh, so it's not an R6S problem but a personal preference issue because the R6S is apparently displaying the original (correct) aspect ratio of the content which is wider than that of your HDTV - hence resulting in top and bottom bars.
So you want this kind of transformation without distorting the proportions:
There's your answer on the 2nd screen...CROP the ears of the man.
Yeah, if the R6S does not have such Crop Option readily, then just do manual X%, Y% zoom adjustment.
What I mean by full screen is no black bars at the top and at the bottom.
So to summarize. 51e450 is native 720p panel. Playing 720p files on the r6s connected via hdmi with video setting of 720p 60hz. R6s menu is shown full screen. Playing the video though is not..