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Check your posture. Slumping posture can cause an appearance of a fatter face by forcing a double chin look. Any fat you do have will simply shift to the most comfortable placement and if your posture pushes your neck and chin downward, this may fatten your face.
Stand and sit up straighter, so as to align your spine properly. Ask for help from a physiotherapist or doctor if you're not sure how you should be standing or sitting to ensure good posture.
Breathe deeply. It is harder to slump when you breathe more deeply, as you need to straighten up to fill with air.
Remind yourself to hold your shoulders back, tilt your head and chin up more and look people in the eyes.
Posture improving exercises exist; ask your physiotherapist for more details or check online sources. Pilates, yoga and similar exercises will help too.