This may simply be due the disparity between a 4:3 aspect-ratio and the 9:16 aspect-ratio of your large screen. The monitor streches the image to the edges of your monitor to make it appear as though it's wide-screen. I don't have a 9:16 monitor myself, but I think it may depend on what format the signal is broadcast in. Does it also stretch the image of other "full-screen" (4:3 ratio) images from other sources like DVDs?

On a side, I once noticed the stretched image on some high-end HD monitors at Best Buy. I asked the clerk why it does that just to see if he knew, and he said he didn't even see anything strange about the image. That kinda goes in hand with my opinion of Best Buy.

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Current:
2014 VW Tiguan SEL 4Motion 43,000 miles.

2016 Hyundai Santa Fe Sport (wife's).

Past:
2006 Jetta TDI 135,970 miles. Sold Nov. '13.
1995 E-320 Special Edition. 220,200 miles. Sold Sept. '07.
1987 190-E 16 valve. 153,000 miles. Sold Feb. '06.
1980 300-D 225,000 miles. Donated to the National Kidney Foundation.
1980 240-D manual, 297,500 miles. Totaled by inattentive driver.