In this scenario, stunting is most usually a result of insufficient WC's and accumulation of growth-inhibiting hormones released in the water by fishes - large fish secret more of this hormone and the usual water changes in a smaller tank don't keep up. To stunt a fish by having it in a small tank, my understanding is that the tank has to be real small, for instance, for instance one would have to keep a 1-foot fish in like a 1'x1' tank or some such horrifying dimensions.
I attribute your fish's growth spurt to better water, not so much to more room, albeit the latter could be of help for sure - every fish needs room to swim and excersize.