What's up with agencies and size?

On days when I've been stood up by a client (again) I often think about agency work. There are always ads around for girls wanted. Every now and then I inquire. Polite emails that mention my size. To date I've only had one reply with a curt "We only take models to a size 10".Sometimes I mention a bit of independent work. I'm not sure if that's a bad thing - maybe they think I'm just gonna steal