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Look in the mirror. If you have a normal size body but feel that your face is not slim, consider whether you might have a distorted notion of your proportions.
Look at closely related family members. Do they have the same facial proportions as you? You may have a genetic predisposition to a certain layout of weight around your face. Also, some forms of facial chubbiness may be related to your youth and "puppy fat" tends to pass with aging.
Ask people you can trust to be honest to you. Do they think your face is chubby or do they think it is appropriately proportioned?
Is your face "puffy", as opposed to being fat? Puffiness can be caused by poor nutrition, salty foods, lack of sleep, alcohol or drug intake, lack of exercise, etc.
Talk to your doctor. He or she will be able to reassure you as to what facial fat proportions are appropriate. Also, your doctor can discard possible problems such as edema (swelling).