When people say that the camera adds ten pounds it’s because cameras used in filmmaking/TV production have a wider focal length and therefore subjects look wider or bigger. Whereas cellphone cameras have a short focal length that makes subjects appear thinner or smaller.

Nowadays, with DSLR’s and a variety of lenses, we are able to depict a wide range of focal lengths by using one kind of camera.

So that is why most people on social media may seem to look thinner than they do in person (especially in selfies because the front facing camera on phones especially have short focal lengths).

And that is also how the phrase “the camera adds ten pounds” came about.