When taking selfies, because the lens of the mobile phone is wide-angle, when taking pictures of yourself, in most cases, the face will be placed in the middle of the picture. The wide-angle lens has the effect of stretching the edges and gathering in the middle, so the face will be smaller.
As far as professional photographic equipment is concerned, different focal lengths will have different distortion effects, that is, the shorter the focal length, the stronger the stereoscopic impression, the longer the focal length and the flatter the picture. So under normal circumstances, after the focal length exceeds 50mm, as can be seen from the figure below, the longer the focal length, the fatter the face.
Because stars have to live under the lens all the time and don't know when they will be photographed from a strange angle, they can only keep their bodies much thinner than ordinary people and dare to walk under the spotlight and lens at any time.