Stopped growing?

One of my rescued kittens seems to have stopped (or slowed way down) in growing... There were three which were way smaller than the other kittens, but two of them seem to be starting to catch up while the third has stayed tiny... I've been giving him extra food and Vitacal. He is very active and good appetite but skinny. My vet said the kitten might have a congenital defect. They have all been wormed twice and are on antibiotics.
Has anyone experienced this or have any tips?

I thought one of mine was not growing until they were weaned recently. Now he gets more milk from Mommy and is growing now.
Could the kitten still not be getting enough to eat? The other thing is that he could just be a small kitten who is going to be a small adult.

Have they been dewormed for tapeworm too?

My vet gave me Drontal which is for tapeworms as well as other types.

I weighed the kittens today. The little one is only 14 ounces, the next smallest kitten is 21 ounces and the largest kittens are over two pounds (one of the largest is almost 2.5 pounds)... They are supposed to be 7-8 weeks old now but the three smaller kittens could possibly be younger than the larger kittens... Still, the two other small kittens are way bigger than the tiny one who does not seem to be growing much at all.

I am having a similar issue with one of mine. The others are so much bigger and advanced and Renzo the runty still has ears that are down.
I am at loss for a reason.

Here are some photos of the small kitten and one of the larger ones, to show the size difference.










In this photo you can see how much thicker and longer the larger kitten's legs are: