Being in the 15th percentile does not mean small. Being in the 95th does not mean healthy. What matters most is consistency — a baby who stays at the 10th percentile is growing normally.
The 3 charts to track: Weight-for-age, length-for-age, and weight-for-length. Head circumference is also tracked in the first 2 years.
Always discuss your baby's chart with your pediatrician rather than comparing to other babies.