My little guy eats 4 cups a day, 1 cup 4 times per day, but he is pretty tall for 3.5 mos. Ribs are easily felt with very little fat.
Just like people, each dog's metabolism is different so each needs a different amount of calories. As long as she doesn't look too gaunt, and doesn't act lethargic or run down, I wouldn't worry about it. If you do have a concern, consult your vet and let them tell you if she is getting enough to eat.
I would almost never recommend leaving food down for the dog to eat as it sees fit. But there are exceptions. As long as she is not overeating, this shouldn't be a problem. In fact, I would be more worried about a pup getting too many calories as recent studies have shown this to be a possible contributing factor to hip problems later in life. At one time it was thought to be fats, but that theory is no longer as accepted as it once was.
If her low consumption becomes a problem, again consult your Vet and allow them to suggest some higher calorie diets. Remember, it isn't about the mass that she eats, but about her caloric intake.