Overweight dogs should be on a diet until they reach their target weight. Your dog’s weight loss should be gradual and can take several weeks. Healthy weight loss plans allow your medium or large dog to lose between one and five pounds per month. Small or toy breeds may lose less than one pound of weight over a gradual period. Even when your dog loses weight, you may be able to increase their calorie intake slightly. However, you shouldn't ditch the new healthy habits you’ve developed with your pup.