Mochi the giant shih tzu

So Mochi was scratching his ears. I took him to the vet. He has an ear infection. Poor kid. But what was even more astounding was his weight. He weighs 15 lbs and he's only 6 months old. He was only suppose to be 12 lbs total. The vet thinks he could be as much as 20-25 lbs full grown. She says to me, "We have 1 or 2 other GIANT Shih Tzus that come in." WTF?! Mochi is a giant!