That's crazy I just measured mine and was furious that they weren't 18" yet. I'm up to 605lbs on Donkey calf Raises for 23 reps and 425 on standing barbell raises (barbell across your back and you stand on your tip toes and come down and just barely touch your heels to the floor). I'm also training them at every workout and doing DC stretches... stubborn bastards.
Congrats, 18 is a good size.