Showpug is right, dogs are healthier when they are lean, Ralphie is 112lb of solid muscle and he is a boxer with a really skinny waist!
The best way to tell if your dog is at the right weight is to run the back of your hand over the dogs ribcage from head to tail, you should be able to feel all of the ribs with a decent bit of covering (more so at the head end). If the ribs are really protruding your dog is underweight and if you can hardly feel the ribs, your dog is overweight.
Just out of interest, what type of dog do you have? Some breeds are more naturally skinnier than others...I mean, look at greyhounds!