My Weimaraner is very small, why is this?

I have a 10 mo old female Weimaraner, she is very small! She is only about 42 lbs, and has been this size for about four months. I don't think she is going to grow anymore. Female weims are usually 65lbs plus. Every other weim I have met is at LEAST this much usually near 80-90lbs. I adopted her from a family, she came with papers. I have talked to them and they don't know if she is the runt of her litter or not. Any ideas? She goes to the vet regularly and they have confirmed she is about full grown, and don't seem concerned about her size. I'm just wondering if anyone has any opinions about my petite little girl.