Weight gain is the result of an increase in body fat. This is usually caused by eating too much, especially when combined with lack of exercise. But there can be other contributing factors, too.

Age
Older dogs are less active, have less energy, and require fewer calories, which is why nutrition formulated for his age is vital to his weight and overall health.

Breed
Some dog breeds are more likely to gain weight

Neutering/Spaying
Clinical studies have shown that the basic metabolism of neutered dogs is lower, and they require few calories

Medical Problems 
Very occasionally weight gain is associated with a medical disorder that may require specific treatment

Overfeeding
Dogs with unlimited access to food understandably eat more than they need — this includes table scraps from family members

Overeating
Many commercial foods are loaded with salt and fat, which can improve taste but make your dog want to gorge

Exercising with your dog

As well as feeding your dog the correct nutrition, promoting regular exercise will help the process of healthy weight loss. Here are a few workout tips for exercises you and your dog can do together:

• Power dog walks with intervals Take your dog for a walk on a leash. Throughout the walk, mix in some intervals of jogging, running or high stepping to help increase your heart rate and burn calories for both you and your dog
• Dog squat tease Stand with your legs spread shoulder width in preparation to do a leg squat. As you descend, tap your dog with his favourite toy. As you rise, lift the toy above your head to encourage your dog to jump after it. This can be done in your home and outside as well.
• Dog tag Just like you did with your friends as a kid, play tag with your dog at your local dog park, in your backyard or even inside your house. You’ll both get a great cardio workout as your dog tries to chase you down.