If she's acting like a kitten, I wouldn't worry. My punks from hard circumstances--starvation, disease, disability, etc.--seem to stay a lot smaller than a sib from a healthier beginning. Since your baby was a homeless child, she might've had stuff in her background that doesn't show now. Sassy, who's mom had 6 babies with mastitis, hemobartonella, etc. is a sole survivor and is quite healthy at 4 pounds full-grown.