Why do I look different when I take pictures on my iphone horizontally and vertically?

So I realized that every time someone takes a video or a picture of me, I end up looking really short (though I'm already short to begin with) and my legs look very chunky and look less like they do in the mirror. But if I take a picture horizontally, then everything looks kind of proportionate and more importantly, my legs are a lot more nicer looking and slimmer.

Is there really a difference between taking pictures horizontally or vertically or is it just me?

Yes, the lens distorts images at the edge of the picture. So if your legs are close to the edge of the frame, they will look disproportionate. Try putting more distance between yourself and the camera. This will help make your features look more true to scale. The type of camera and lens also change your appearance based on how you use them.

I would venture to guess that the images that you've looked at were all taken with a smartphone. Smartphones all use wide-angle lenses. All wide-angle lenses produce distortion. The more the camera is tilted up/down, the more obvious the distortion. Also, the distortion becomes more pronounced the closer you get to the lens.

This is not the result of a poor quality lens, but rather the short focal length. Using a longer focal length and stepping back will reduce or completely eliminate the distortion.

Picture quality depends on the skill of the photographer.

If you know exactly what you are doing, camera orientation shouldn't affect how you look and the mirror would agree.

Probably because you have to hold the camera further from your body in order to get the whole body in using horizontal mode. Any camera held too close to the body inevitably distorts body shape - look at most selfies with huge noses.