I don't understaNd why this happens?

I look in the mirror and see myself the same way I do in a webcam or the cam on my phone but then I take a picture with a normal camera(nikon) and I look completely off. For one it makes me a good deal larger everywhere ( including a wider face, waist, etc) people have even told me while my face still looks pretty, alot of pics taken with the cam distort what I really look like? So what gives?
This is making me super self conscious whenever I see a blimp size version of myself in the cam( I know a bit of exageration but how I feel lol) so can someone explain?

Try backing up and zooming in. When you hold the camera too close (e.g. At arm's length), then lens distorts your features - it just a basic fact of photography physics.

If you study the Physics involved in glass properties and optics combined with a slew of electronic limitations, you would start to understand why that happens.

Adding to other comments, one issue could be the focal length of the lens when you are shooting. If it's a typical compact digital point-n-shoot that is set to it's widest setting, this is an unflattering focal length similar to a 24mm, 28mm or 35mm lens on a DSLR. These wide focal lengths are almost never used for portraits. At a minimum, a full body portrait (head to toe) would typically be shot with 50mm lens on most consumer DSLRs or an 85mm lens on full-frame models. Without requiring you to take a classes in physics to understand how lenses work, the reasons boil down to the fact that shorter focal lengths (wide angle lenses) have inherent distortion that causes things like your face, waist, etc to look wider than normal. Webcams have fairly short focal lengths too but, your distance from such cameras is actually quite long relative to the focal length of the lens and you are typically dead center in the middle of the frame so the distortion is less apparent.