Cameras make a flat image, and you are not flat. Since some distortion in necessary, its a question of interpreting the distortion.
Wide field of view camera lenses focus in a hemisphere, sometimes this produces a pronounced “fish eye” effect but the effect is always present. Things closer to the camera are bigger than things farther away. That makes your nose and belly look larger (more pixels per inch of you) than your arms.
One solution is to use a narrow field of view lens, from farther away, but clearly that’s not happening in a selfie.
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