If you're concerned after doing a body check, you should take your cat to see a veterinarian. You can weigh your cat yourself at home but it's better to take him to the vet. The scales at a vet's office are specifically designed for use on animals. A vet will also be able to determine if your cat is medically obese considering his body type. While a body check can give you insight as to whether you should be concerned, professional veterinary evaluation is important when diagnosing your cat as overweight or obese. When you're at the vet's office, your vet will ask you a variety of questions regarding your cat. This helps determine whether your cat's weight is due to environmental or medical factors.  Obesity in cats can be the result of environmental factors. Your vet will want to know how often you feed your cat as overfeeding can lead to weight gain. If your cat hides a lot due to fear of children or other pets, he may not be getting enough exercise. Your cat also may be bored and not have enough toys or entertainment. You might have to adjust your home environment to help your cat lose weight.  However, environment is not always the cause. Some medications, diseases, and medical conditions can cause your cat to gain weight. If your cat has any other troublesome symptoms, like vomiting or diarrhea, let your vet know. He or she may want to conduct certain tests or lab work on your cat to rule out an underlying medical problem. If your cat's weight gain is environmental, talk to your vet about how you feed your cat. He or she can suggest changes you can make to get your cat to a healthier weight. It's important you talk over major changes to your cat's diet with your vet beforehand.
Make an appointment with your vet. Determine the cause of your cat's weight gain. Discuss your cat's diet with your vet.