Young Pregnant Mama and Stunted Growth? Are There Any Health Concerns?

Hello! Because apparently our holidays aren't complete without another member, we've decided to adopt a sweet, lovely 6-9 month old blue domestic shorthair (all markings point to Russian Blue, but of course we don't know as she was abandoned). She's tiny enough to fit into two of my small girly hands, and just a little lovely charmer. If you're TL;DR question is below in bold.

Background: We can't bring her back to our home for four more weeks because she has two two week old kittens. Whoever abandoned her left her heavily pregnant and with a dislocated/broken tail (one thing at a time for this poor girl). Despite everything she's been through, she has a great temperament and we're going in at least once a week to her foster home to get her acclimated to us.

Apparently the vet told the foster home that she won't grow any bigger (or maybe only a little bigger, but sounded doubtful), because she had kittens at such a young age? Is this true, and can anyone shed some light on this?  

We don't care how big she is, but I was really curious about it, and I just can't find any info about it other than it might be linked to calcium deficiency. But that makes me think if she's fed a well balanced diet, surely she'd bounce back in size? I'm pretty well-versed in cat basics, but this threw me for a loop.

Are there any health concerns associated with her being stunted we should prepare ourselves for?  We're preparing the house for her arrival, and I'm just trying to think of any special needs she may have.

Thank you so much! There's so much wisdom here that I figured I'd ask.

PS - if it matters, we do have another cat, but we've got a safe room picked out and are pretty well-versed in cat introductions, so we're not super-concerned about that. Slow and steady does it.

Oh the poor baby. Having kittens while she's just a kitten herself! And she's so little too. How people can be so irresponsible, I just don't understand. Anyway, it's great that she was rescued and fostered and now found a forever home with you. She is adorable.

I have not heard of this either that she will be tiny forever. I would think with good nutrition, she will grow a little more since she's still young despite her having kittens. I think if she's healthy now all you can do is feed her and love her and wait and see. Personally, as long as there are no health issues, I wouldn't mind if she stayed little. She's just so darned cute.

I'm inclined to agree with you about nutrition. I'd never heard of this until now, but then, I'd not known such a young mother before now, either. That's why I became fascinated abut the topic, but I can't find much of anything on the net. My Googlefu has failed me...

If she stayed small, it'd be perfect symmetry with my 8kg monster cat LOL. I just don't know much about the whole topic, and I like learning, for learning's sake and to be a good meowmy.