If you are a female character you can eat nothing but donuts,burgers,and pizza and work a desk job but still weigh in at only 120 pounds or so.
If you are a male character you can have forty pounds of muscle despite the fact that you will never be seen once lifting weights and you character may be kind of a slob who eats nothing but junk food and and has a job like a doctor or a writer. Unless you have a failed marriage and a kid who never talks to you,then you will be fat.