I've noticed that the average cat (or just the girls I've had, lol) grow UP until about 2 years of age, and then they fill out (they grow sideways, lol), gain muscle and look more "put together" (versus the awkwardness of adolensence, lol)
Yep they grow plenty after 6 months, my cat went from 7.5 lbs at 6 months to 11.5 lbs now at 20 months, plenty of that is muscle but some of it is size, btw she used to be 12lbs and is a little underweight now so none of the weight gain is fat.
My Jack's only 6 month's old and he's huge compared to what Rosie and Sophie were at that age, so goodness knows how big he'll be when he reaches adult status