Some snakes just don't grow as big as others. Yours sounds like it's probably just a small male. Do you have temps between 90 (hot side) and 80 (cool side)? If it's colder, that might be why it hasn't grown as much and doesn't have much interest in eating more frequently.
Bigger isn't always better.
If your snake looks healthy and at a good weight, acts healthy...I wouldn't worry.
~jenny
PS: I got my nice reduced pattern female from Petco. But then, I worked there at the time and probed the snakes I liked to check genders. 

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