I go by rat weight and a heavy small or light medium is not a big difference.
Most of my snakes get one medium a week.

Small is 46-79g
Medium is 79-149g

Are you underfeeding? Depends on the snake.................. if they want to eat I feed and if they turn away they don't eat.

BTW my sons 2 year old redtail is about 3000g and downs 2 to 3 mediums rats a week and most of my balls are on smalls within 5 to 6 months if they eat when offered.