It is typically recommended that you keep the puppy on the food the breeder sent her home on for about the first year. I'm assuming you did your research and chose a breeder that you trust, who knows her lines and what type of food they thrive on and whose advice you trust?
Getting on the food rollercoaster for a puppy is a bad idea. Find something that works and stick to it. I would not change to Science Diet, which is no better than puppy chow, but either go back to what the breeder had her on or switch to something similar but perhaps "higher quality" such as ProPlan. Many people, including myself, raised their healthy puppies on ProPlan Sport 30/20 and many people, including top-notch breeders, feed this food and have excellent results.
A quick Google search will bring up all of the issues that people face with Blue Buffalo (soft stool often being one of them) as well as the lawsuits from being a shady company, lying about their food to consumers. It's probably one of the last foods I'd ever feed my dog. And Science Diet? It's very expensive for what you get and the ingredients are nothing to write home about. Similar to Blue Buffalo, they have good marketing. If you're looking for something with less filler, this is not the food to feed.
How big were the puppy's parents? Lab puppies run the gamut. If she looks healthy, she should be fine. The bag instructions are a guideline. She is 14 weeks, so in two weeks when she is 4 months old, weigh her. Double that and you should have her approximate adult weight. She sounds like she is going to be on the smaller side, so the amount you're feeding her sounds fine. You may go up in food amount as she grows and becomes larger and more active, and then you might come back down when she is an older adult. Just depends on the dog.