Mine is ten inches after two years, and that was mostly on algae wafers. Since I started feeding bloodworms he's grown about a half an inch. Clare's gobies were up to 20" in one year, but that was on her special diet (and by that I do not mean she was feeding them table scraps). I'm hoping Ying's new diet and a brand new 125 gallon tank will boost his growth.
At that size, your goby never displays at you? Maybe it's a female. Here's a neat thing you might want to try. If you put your hand into the tank, and try touching the fish very gently. The first couple of times I did that it scared Ying and he moved, but then he started approaching my hand. When I pet him, he lines up by my hand and ungulates against it, like to individual gobies might do. Is that how the two big gobies acted?