"Knowing that Rat snakes generally hatch at 10-14 inches in length, and have a predictable growth rate, "

Not true at all, actually. I have some yearlings that weigh in at 50 grams right now. But I've also had yearlings that weighed over 100 grams. One of my females is also what's referred to as a "linebacker" in that she is a short, thick-bodied individual. She's 3 years old, and about 200 grams now.