It is the biggest problem for a fat person to take a selfie in which he looks better . It's difficult for a fat person to take perfect selfie .
The old cliche pose that rotund people used to take selfies was to raise the camera above eye-level pointing downward. Perspective makes the face appear relatively larger, and the body smaller, than it normally would. The larger the person, the higher the camera would be placed. The pose has apparently fallen out of fashion.
Instead of raising the camera higher, a variant pose is to lean forward toward the camera. It works the same way. Perspective makes the head appear relatively larger, and the body relatively smaller, than it normally would.
There are various other tricks that are not specific to selfies, such as turning to the side, lifting the chin away from the neck, reducing kyphotic posture, using appropriate colors and patterns, etc. See What are the best techniques for photographing overweight people?
Perspective can also be used to exaggerate or minimize other body parts. For instance, why do certain celebrities appear have such huge buttocks? In addition to implants, they place the camera down low and project their rear toward the camera to exaggerate its size.
Controlling the angle of the primary light source illuminating your face can go a long way to making your face look a bit slimmer. The last thing you want is for on-camera flash to be the primary light source. Rather, find a bright light source that allows you to stand next to it at about a forty-five degree angle to your right or left, with the light just a tad higher than your face. The resulting shadows in the image will tend to minimize the edges of your face and make it look slimmer.
Another thing you can do is extend your neck forward and raise your chin when looking directly towards the camera. Although it feels unnatural, it looks better because the skin under your chin is stretched tight instead of hanging more loosely.
Don't take a selfie then. Seriously. Digital cameras, particularly selfie cams, have comparatively wide angles. It's a well-known rule of thumb that favorable portraits tend to be taken with 100mm (35mm equivalent) lenses from a distance of several meters.
A "selfie" tends to be distinguished by the photographer holding the camera, making this not a good proposition. The proper solution is a tripod (or other fixture for phone and/or camera) and a self-timer. That also stops your arm from featuring prominently, distorted and blurry in the picture.