Here is a picture of my "teacup" stingray! She is a 3 year old retic in a 240 gallon tank! I remember when she was only 3 inches in diameter when I got her. 
I think with the very good water quality where I live and the 50 percent water changes I do 2 times a week is the key. They also get fed a variety of food and I think that also helps!
What do you feed her for a variety? My teacup is currently 8 inches on red worms and carnivores pellets.