Is it abnormal for an 8 month old Sav Monitor to only be 10" in length?

My Savannah Monitor is 8 months old and only 10" in length. I feed him every other day, either one hopper mouse or as much insects as he will eat. However, he's only 10" long and I'm reading all these other people saying their monitors are 12" at 5 months old, even some that are 2 feet by a year old! Am I doing something wrong? D: I was told that Sav monitors reach full growth by about 3 years old...but still, is mine too small for his age?