Some useful info:
Some veterinarians recommend the following: Record an accurate pre-diet weight. Reduce by one-third your pet’s total daily ration previously given. Include in this total all treats, snacks, or left-overs if you insist on continuing to provide these. Reweigh the pet in 2 weeks. (Remember if the pet begs for food, that's a good sign! But don’t give in.)
If you find upon weighing your pet after two weeks that it has lost even a little weight, you’re on the right track; keep up this schedule! If no weight loss is evident, again reduce by one-third the amount being fed. Weigh the pet again in two weeks.
Depending upon the results either keep feeding this amount or reduce again by one-third the total amount being fed. If you persist a good outcome is certain. Many veterinarians believe you should not feed the "Reduced Calorie" or "Lite Diets" or "Senior Diets"! These diets have very restricted fat levels to reduce the calories but by necessity have increased the carbohydrate percentages. This increased carbohydrate stimulates additional Insulin secretion which tells the body to store unused calories as fat! There are a multitude of overweight dogs that have actually gained weight on those "Reduced Calorie" weight loss diets.
It is also quite important to get everyone’s cooperation in restricting the pet’s intake. There is usually someone in the household who feels sorry for the dieting pet and surreptitiously provides "just a little" something extra. More helpful would it be for the person to take the pet for a walk or a run to burn off a few calories.
Useful guide to whether your pet is overweight and the health dangers on
Pet Food Choice - overweight dogs and cats - the problem of pet obesity