Kathy Jinkings, Aquarium Fish, Collins 2000, has a good quote on this one:
"It is often rumoured that fishes will grow to the size of the tank they are in and then stop. This is absolutely true. The stopping process is also known as death, and the fish will indeed grow no more afterwards."
It is, in fact, quite a complicated subject and poorly understood. One school believes that the body growth will be stunted but the internal organs continue to grow; this is dubious.
More likely, it is believed that some fish can have their growth stunted by sending out special stop-growth hormones if the tank is too small (alternatively not producing enough growth hormones); this may only apply to certain species. It is a well known fact that many species grow much bigger in the wild than in captivity; this could be a reason.
Another reason could be that their bodies do not develop due to lack of exercise and/or appropriate food (think Roumanian orphanages). Stress is also likely to inhibit growth (in your tank and in the orphanages).
Lack of space is also likely to lead to illness, which again may prevent full development.
And many fish take a good many years to reach full growth; if they never survive that long then you won't see them at their full size in the tanks.
If a fish's growth is inhibited due to any of the above factors, then this is clearly not a happy outcome or a success for fishkeeping.
It is also true that some fish do keep growing long after you would have thought the tank would have made it impossible; they end up deformed (think Chinese ladies feet) and eventually dead.
I imagine (hope) your situation is that you wanted to keep your BGK in a little tank at first and then transfer it to more suitable accommodation as it grows; so your main worry would be if there is anything in the growth hormone theory. I'm afraid I don't know and I'm not sure anyone does know. There may be other reasons against it too- but I do not know enough about BGK's.
