Weight does play an important role in a dog's health. Some breeds and mixes are prone to weight gain. Excess weight can play hell with joints, leading to early onset hip dysplasia. It can cause arthritic back pain in extra long dogs like Corgis and Dachshunds. Heart disease and breathing problems can result. A vicious cycle of overeating and lack of exercise from a lethargic lifestyle can lead to a pet not being as happy as they should be.
If you own a dog, and your dog is fat, take a look at yourself. You are in total control of it's diet. Use a measuring cup or a tin can to dole out the kibbles, and cut back on the amount. Send the table scraps down the garbage disposal. If your dog is fat, it's because you feed him to damned much. He doesn't need dope. You need to stop being one.