She might just be a small female. Any info on her parents size? Pictures would be helpful. Top over view and standing side view. Breed standard for females is 48-70 lbs. She won't fully settle into her adult weight for another 6-12 months.
My female stayed a steady 65-67 lbs (she's taller) from age 1 to 2 years. Between years 2 and 3 she began to put on more muscle and physically mature. Before we knew it she had literally plumped out to 78 lbs. Then the vet said she needs to take off 5-10 lbs. As much as we have tried to take the weight off she seems to stay steady at 78 lbs even though we steadily decreased her food (80% raw fed) from 2.5 lbs per day to 1.25 lbs per day. At 6 she is still 78 lbs.
Point being if she is otherwise healthy/active and vet has ruled out health issues I wouldn't worry too much unless she is substantially underweight.
This body condition chart might be helpful