If your pair is constantly breeding, that will slow down their growth rate quite a bit. I finally separated my pair and my male's growth shot up since he finally stopped worrying about babies and flirting.
They do grow quite slow, I got my male in 6/07 at 1.5" and he is only 4.5" now.
lol they are like hamsters, however dont look down on the breeding, i mean there awsome parents and do all he work for you, however u shud hope they slow down breeding cause in heat they get vicious