If your baby is breastfed, but doesn't seem to be getting bigger, it could be that your milk supply isn't adequate. This can be a result of stress or fatigue. You can talk to your pediatrician or a lactation consultant for some tips to help boost your milk supply. If you're formula feeding, a slow weight gain could be due to problems with digestion or measuring the formula incorrectly. Again, your pediatrician or lactation expert can let you know for sure.