i dont see how its not possible?
the AVERAGE growth rate, meaning not perfect water parameters/water changes, is 1 inch a month, up until they are around 6'' mark.
so if he had it for a year, it got around 6'' in the first 6 months, and 2 more '' in the next 6 months. very likely.
if your rbps are not moving, their stressed. mine swim constantly , but i have a powerhead in there to create some current. if you want to notice some growth, do daily water changes for a while. you'll definately notice their growing no matter what size they are.