It is hard to say. Just as Labs turn out black, yellow, or maybe chocolate, but not a mixture, puppies usually turn out about the same size as one parent. The one rule is that the adult will twice as big as it was at 3 months. You say she is 15 pounds and the father is 120 pounds. You didn't say if you can feel either's ribs. If she gained 5 pounds in a week, likely your are feeding her too much. If you are feeding a concentrated chow like most of the more expensive brands, you should only be feeding her 2 cups a day or less broken down into 3 meals a day until she is 4 months old. My 8 month old and 11 year old both are getting about 3 cups a day of Pro Plan. Over feeding is a big problem with Labs. No matter how much you feed most Lab puppies, they will snarf it down and look for more. Unfortunately it leads to excessively fast growth, worsening genetic tendencies toward joint problems, and shorter lives for obese adults.
You should be feeding a puppy chow now. Switching to an adult chow for the
larger breeds at 4- 6 months, slows growth and helps develop sturdier
joints.
Your dog, even young puppies, definitely should be narrower at the waist than the hips and chest. You should be able to easily feel the ribs, but not see them. Each dog is different. Standard recommendations are a good place to start, but each dog must have its food adjusted to its individual needs. See
http://www.purina.com/dogs/nutrition.asp?article=292.
The dry dog foods are actually better for the dogs. It helps keep the teeth
clean, and exercises the jaws and teeth. If you feed a complete dog chow, you
shouldn't need to give the dog anything else. Various people foods may only
add more calories without providing proper nourishment. Look at how many of
us are too fat too. If you don't want to pay name brand prices look for
certification of passing the AAFCO tests. Make any changes in diet slowly
over about a week.
The premium, concentrated foods give you the assurance of a brand name. They
produce a smaller, firmer stool that is easier to clean up. It is easier to
maintain weight on a finicky eater. The lower cost foods may actually have
lower fat and be better for your dog. Many dogs would enjoy eating the larger
portions you have to feed.
Here is a link to a picture of a Lab puppy dissatisfied with his healthy build,
http://www.photolocker.net/images/La...theproplan.jpg