Question about pregnant cat

I have a small cat who I rescued as a pregnant stray in March. She had her kittens on March 29th, only a couple weeks of bringing her into my home. She had 6 kittens, 2 males and 4 females. I gave the 2 boys and 2 girls to loving homes when they were about 6 weeks old (they were already weened) since I am only allowed a certain number of animals on my property. Now, I took the mother to the vet when I first brought her inside. He told me that she's had other litters before when she was still little and that she's roughly over a year old. My question is since she had kittens when she was so young if that affected her overall size? Like I said, she's a small cat. She is a Maine coon mix and she's maybe half the size of other Maine coon mixes. I want to know this because I'm keeping two of her female kittens and I wanted to see if they'd be her size or larger. Of course, I will still love them no matter how big or small they are. I'm just curious on how big they'll be. 

 ~ Thank you! 

Thank you for rescuing a stray.  In the future, it is really best for kittens to stay with mom for 10-12 weeks.  Even if they are eating cat food, they are still learning a tremendous amount about how to be cats and it's important to their emotional development.  Were you able to rehome them in pairs?

Yes, it's possible that her growth was stunted.  Kittens will take nutrients from a mom cat that otherwise should have been going to her body.  This is especially damaging if mom was still growing and if she was a stray and not getting enough nutrition period.

Have you been able to get mom and kittens spayed?  

We'd love to see photos of the ladies!

Yes, they were all adopted in pairs. I wouldn't accept anyone who wanted just one, I didn't want to put that kind of stress on them. I'm getting them spayed as soon as I possibly can. But, somehow the remaining kittens went back to nursing before she could dry up. My vet refuses to spay the mother until she's dry. I've already tried separating them but the mother was just crying and crying and she wouldn't eat. It was too much stress on her. I decided to keep them together and let the mother dry up on her own. They're all indoor cats and I don't have any non-neutered male cats, so I shouldn't have to worry about pregnancy for now.  

these are the two kittens I'm keeping
and this is the mother. (She isn't fond of having her picture taken

She's a beautiful tortie!!  Hoping you can get her spayed soon.