I have had a similar experience with one of my TSN. He's probably 17" and about 16 months old. While this is still huge growth rate, its no where near my other TSN and the RTCxTSN. The bigger TSN is 25", is at least double the mass of the smaller TSN, but was only about 2.5" bigger than the smaller TSN at the time of introduction. There was no competition between the two that I could see. They hung out all the time, and there was always plenty of food. The RTCXTSN grew on the same growth rate as the big TSN and is about 23" now. The smaller TSN almost stopped growing for a couple months at about 14", and then went through a little growth spurt and hit his 17" or so.
The only difference between the fish that I could see is that the 'little' guy does not stuff himself like the other two. He eats just until he has a fairly good bulged and then stops. The other eat themselves silly. Even if there is left over food in the tank he's not interested. It does make me wonder if there is some anatomical difference in his GI tract.