thin salmon boa

[slick_slider900]

Hey got a few questions.I just bought a female salmon boa from a retile expo here in CO. and it seems to be a little thin to me. She is about 4ft. long and is currently eating small rats. So given her length and approx. date of birth which I figure is late 09 how much should she weigh? If she is under weight how should I with the exception of just feeding her more could I promote weight gain?

thanks....the Griz.