Personally I like my pets a bit chunkier, not fat, but a bit more ....err....well....'fluffy' in the middle. lol

If the vet has examined the dog and found her to be healthy and isn't concerned then it's probably just a matter of your preference in appearence. Vets see WAY too many outright fat and obese animals that have so many health problems from diabetes to heart problems that seeing a normal weight or even slightly thin animal is almost a sigh of relief.

If you want to put a small amount of weight on just increase the dog food by a very small percent and let the dog get some good excercize to build muscles. Don't over-do the weight gain though. It's not easy to get weight back off, the animal isn't happy about it and there are a LOT of health risks with overweight animals.