Hindi na kailangan mag-alala kung masaya at healthy ang baby mo, pero I'd suggest asking your pediatrician pa rin during the next check-up kung normal lang ang weight gain niya, basta siya ay active, kumakain ng maayos, at walang health issues, most likely okay lang yan! A few of my mum friends had babies na ganyan, and their pedia said it's just genetics or kung naturally petite ang baby; what matters talaga is the trend ng weight gain, hindi lang ang number mismo.