I was just talking about this the other day, dragons reaching basically adult size by 6 months. If you feed them enough, they will grow that quick. IMO, it's better to not allow them to grow that quick, by limiting their food intake if need be, but it's not unheard at all.
I kind of agree with AHBD, but I'm more towards the camp of, no real GG genes exist anymore. They are all too diluted IMO to have any real effect. Large dragons produce large dragons (generally) but the true traits of the GG aren't seen anymore.
Like the other posters, I'd be interested in seeing a picture and knowing the length.
-Brandon