This may depend on what you usually eat. Eating too much carbohydrates is easy to gain weight, while eating too much fruits, vegetables and meat is not easy to gain weight. Moreover, you may lose weight and make your body thinner and less likely to gain weight. Eating more and getting thinner may be related to the stomach. Generally, people with a bad stomach will lose weight instead of absorbing it. You can go to the hospital to consult a doctor if necessary.