When my baby was of the same age, I also used to think so and tried ways to make him look chubby.... Then my son's paediatrician guided me and said that chubbiness is also dependent on the genes... So he told that what mainly matters is that the pee count should be more than 6, should achieve milestone on time and be active.... These are signs that the baby is healthy... And believe me when I followed my doctor's advice it brought peace in my mind and helped a lot... So I would suggest you that since the pee count is good, and if the baby is active with timely milestones then don't worry about it dear....Happy parenting