Well, IMO, if you can see all her ribs and people are constantly commenting on how thin she is, then you do have a problem. Have you tried feeding her more? My bitch gets about the same amount of food as her brother, even though he weighs 25% more than her, because he's a lazy so and so whereas she's very lively and inquisitive and on her feet much more than him. If your girl will eat more, I'd try increasing the size of her meals and sneaking her a few high-calorie snacks during the day and see how she goes. Personally, I never take any notice at all of the amount of food my dogs are "supposed" to get. I just look at them every day and if they look a bit thin I give them slightly more and if they look a bit fat I give them slightly less. It works for me .