Zoom a 2.39:1 film to fill a 16:9 television and you throw away about a quarter of the width. The arithmetic is not close: 2.39 divided by 1.78 is roughly 1.34, so the picture has to grow by about a third for its height to reach the top and bottom of the panel, and the extra runs off the left and right edges. Those edges are where a widescreen composition puts things on purpose.
The trade is worse than that, because what stays behind is softer. The set is now scaling a smaller region of the source up to fill the whole screen, so each pixel of the film is stretched across more pixels of the display. Nothing has been recovered. Something has been cut off and the rest has been degraded to pay for it.
The bars are not wasted screen and they are not a streaming fault. Your television is 16:9, which as a ratio of width to height is 1.78 to 1. A great many films are finished at 2.39 to 1. Put the second inside the first and there is space left over above and below, and that space is the shape of the film.
What the zoom button actually does
Televisions offer a control called Zoom, Fill Screen, Wide, Stretch or Auto Wide depending on the brand. Selecting it removes the bars, and people reasonably assume it is recovering something.
It is doing the opposite, and it is the reason Filmmaker Mode disables overscan as part of its specification. To make a 2.39:1 image fill a 1.78:1 screen, the set enlarges the picture until the height fits, which pushes the left and right edges past the sides of the panel. The arithmetic is straightforward: 2.39 divided by 1.78 is about 1.34, so roughly a quarter of the width goes off the edge of the screen. On a shot composed with two people at opposite sides of the frame, one of them is now partly missing.
There is a second cost. Enlarging the image means the set is scaling a smaller region up to fill the whole panel, so every pixel of the source is being stretched across more pixels of the display. The result is softer than the untouched image, sometimes noticeably.
The bars are the cheaper option by a wide margin.
Overscan, which does the same thing without asking
Overscan is the older version of this problem and it is worse, because it happens whether or not you asked for it.
Analogue broadcast assumed the outer edges of a frame were unreliable, so televisions cropped a few percent off every side and scaled the rest up to compensate. Some sets still do this by default on some inputs. You lose the edges of the picture, and the whole image is slightly soft, for no benefit at all.
Look for Just Scan, Screen Fit, Full Pixel, 1:1, Dot by Dot or Native in the picture size menu and select it. Selecting Filmmaker Mode also disables overscan, because the specification requires it.
When the bars are on the sides instead
Vertical bars either side of the picture mean the source is narrower than 16:9. Older television and films from before widescreen were shot at 4:3, which is 1.33:1, and that is the correct presentation for them. Stretching a 4:3 image horizontally to fill the screen makes everybody wider than they are, which is a stranger choice than it sounds once you notice it.
Some services letterbox their own material before sending it, which produces bars that are baked into the video rather than added by your set. In that case no picture size setting will remove them, because as far as the television is concerned they are part of the image.
The box settings that change the picture underneath all this
If your source is an external streaming device rather than the television's own apps, two of its settings matter more than anything on the screen.
Match content frame rate. Most boxes output at a fixed refresh rate, commonly 60Hz, and convert everything to it. A 24 frames per second film converted to 60Hz gains a repeating stutter in slow pans that no amount of picture adjustment will fix. Turning on the match frame rate option, which Apple TV, Roku, Fire TV and Google TV all offer under display settings, lets the film pass through at its native rate.
Match content dynamic range. The same idea for HDR. A box left converting everything into one format is making a decision the television is better placed to make.
Both of these are off by default on several devices, which is why a film can look wrong on a box and correct in the same set's built-in app. If motion is your complaint rather than shape, it is also worth checking that motion smoothing is off, since that produces a similar sense of something being off without an obvious cause.
The short version
Leave the bars alone, set picture size to the native or just scan option, turn on frame rate matching on the box, and let a film be the shape it was composed in. If the picture still looks wrong after that, the problem is more likely to be brightness and the settings that control it than geometry.
