Why is my cat a midget?

My cat is full grown, at nearly a year old, but she's still as small as a kitten. She was the runt of the litter, but still, I think she should be bigger than she is. She looks like a 6 month old kitten. She has big ears, a little head, and a little body. Should I be worried about her, or is it normal for some cats to stay so tiny?