A fat stomach results from genetics, lifestyle, and some medications. These factors cause excess fat for a variety of reasons:
Genetics
Some people are genetically predisposed to store more fat in their abdominal region, causing a fat stomach and excess fat around the hips and back, known as love handles. This can be the case even if they have a relatively low body fat percentage. Genetics also determine your metabolic rate and influence your tendency to accumulate fat, including the abdominal region.
Body weight and composition
Being in a caloric surplus (where you are regularly consuming more calories than your body is able to use) consisting of highly processed foods and sugar, combined with a sedentary lifestyle leads to excess body fat. The stomach is one of the most common areas that accumulates stubborn fat as a result of diet and lifestyle habits.
Medications
Drugs like corticosteroids, antipsychotics and antidepressants can increase your appetite and slow down your metabolism, leading to weight gain overall, including in your midsection.