Camera roll zooming in on pictures?

I have an iPhone 6s, and my camera roll is doing this weird thing. When I m looking at certain pictures on portrait, my phone will zoom in on the picture to make it fit the whole screen rather than showing the whole picture with the black space on top and bottom. Then when I turn to landscape, it shows the whole picture. I don t know if I m making sense, but basically it will zoom in so that it fills the whole screen, and it looks like the sides are cut off. I ve tried like pinching the screen to zoom out, but it just takes the individual picture down and shows me all my other pictures. Is there some way to fix this? It s really tedious and annoying. Thank you.

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