Not neccessairly, The bigger the tank, the faster they grow, the deeper the tank the bigger they get...
Look how deep the rivers are, thats how they get so big, and that their water is fresh every day....
Most Red Belly Piranhas are from 9-12" but some get bigger and some get smaller.
does diet matter? i fed mine feeders until they got sick one time. since then ive fed jumbomin pellets. they got huge off of them. right around 9 inches at two years old in a 75 gallon tank