Don't worry that's normal. My baby was within 90% for height and weight at birth but dropped to 75% aft he starts walking at 9 months. While he maintains his height, his weight went down further to abt 60% aft 1 year old. As long as baby continues to eat and drink normally and still within the healthy range, it's not much of a concern. Genetics might play a part too, my boy is tall but slim like his father now at 2 year old. Though I do miss his chubbiness!