That's one of the intrigues of fishkeeping, "You just never know for sure until it happens" regardless of what the books or anybody says! A lot depends on setup and food, but yours may have come from unusually small or exceptionally large parents- who knows! My first Tire Track, about 20 yrs ago, never reached over about 14" in the 8 yrs I had him. My current baby has grown from 3" to 16" in the last 6 months. I've already had to move him from a 55 to a 110 because my cories were mysteriously disappearing... I would suggest that if you've already grown fond of him, just wait and see.