Don’t over feed your dog (always follow the directions on the packaging or from your vet) as obesity can lead to all sorts of health problems and ultimately shorten your dog’s life.
As a general rule, if your dog is a healthy weight you should be able to feel his ribs, but not see them (depending on breed and coat type). If you can’t feel them, then your dog is overweight and if you can see them he is probably underweight. Ask your vet if you’re not sure.
If he’s overweight, cut down on in-between meal treats, switch to a ‘diet’ specific low calorie food and gradually increase his exercise levels. Your local vet surgery may run a diet clinic to help your pet lose his excess weight under safe medical supervision.