I read that kittens, especially females, stop growing after they go into heat. Their bone growth shuts of, so if you get them fixed when they're young, they are "leggier" when they're adults.
All my males are bigger than the average domestic cat. My 10 1/2 month old long hair male, Rascal, weighs 11 pounds and he is still growing! He is so long! I think Rascal is a Maine Coon Cross, he is just so huge! My females are all lightweights, except for my Bengal, Majesty, she weighs 8 pounds.
I thinks cats are full grown between 6-9 months, but that differs with certain breeds. Long hairs are almost always bigger than short hairs. They are much longer, stalkier cats.
Your babies probably done growing, but not done gaining weight!