Now obviously, I dont know your dog, but unless she is showing a lot of ribs, it may be possible that she is just a correct sized Labrador. If you look at breed standards, females are supposed to be between 55 and 70 lbs. At hunt tests and stuff I see so many of these big bruisers over 80 and and even 90 pounds sometimes. People get confused about what the dog is "supposed" to weigh. My female is about 4 1/2 years old and she weighs about 57 right now, in the spring and summer when she is training pretty hard, she may go down to 52 sometimes, but she looks too skinny there, too many ribs showing.
Like I said, I dont know your dog, or what her parents looked like or anything like that, just wanted to throw out another something for you to think about. My dog is an HRCH and she always was either the smallest or one of the smallest at all of her hunt tests, when, by the book, she was perfect!