Two littermates, one is much smaller and didn't grow much in a month.

Okay, I am looking for someone to talk me down.  I bought two Maine Coon kittens from a local family breeder.  I viewed the cats at 9 weeks and they looked very playful, healthy and happy.  I brought one home two weeks ago after he got his vaccination, a male (my avatar) and he is everything a cat should be.  His health card from the vet said he was 11 weeks and 2.79 lbs and it looks like he has been growing nicely since birth.  The second one, a female, had to delay getting her shots since she had a respiratory infection.  This worried me, and the breeder gave me the name of her vet.  I talked to the vet, the vet explained that she likely caught something from one of the other cats and so she prescribed an antibiotic since she had a mild fever.   She was well enough to get her shot a week later and I picked her up on friday.  I noticed right away that she was smaller than the other one.  When I got her home and looked at her health card, I saw that she had barely gained any weight in a month, and she was only 2 lbs.  She feels very thin.  Otherwise, she looks really good - very active, curious, and eating like a horse.  She is also voiding normally as well.   I am guessing the weight is because she had the cold and probably wasn't eating much.  I am planning on getting them to the vet this week, but I guess I was looking for any input.  

It's not unusual at all for a litter mate, especially a female to be smaller than the males. The fact that she has been sick plays in too. I have a feeling with good care and a little babying she will catch up, she just had a set back for a while. Females are almost always smaller. If she feels thin, make sure she gets some extra kitten food, it is higher in calories and nutrients. You might get some kitten replacement milk and see if she might drink that, it is a builder upper too. I had a little male that got very sick, to the point where he had every bone sticking out and couldn't breathe, we couldn't catch him to help him until he was so weak he couldn't escape us. I truly thought he would never recover. Well, you should see him now! he went from being half the size of his litter mates to half again as big. He still suffers from respiratory problems once in a while, in fact had a runny nose at the start of this winter. I put him in the heated shop to sleep at nights and he quickly got better. Since your little one is so young, I think she will be fine, she'll just take longer. A vet visit would set your mind at ease, but I think she will thrive in her new environment. All the luck with your little ones, bless you for giving them a forever home!

Just wanted to update that at the vet visit she just had, both cats had gained over a pound in just a few weeks!  The male is now 4.1 lbs and the female 3lbs, so it seems that they are both growing well.  The respiratory thing also seems to have gone away too.  Both kittens seem settled in.  In fact, they have taken over.