I think there's a chance your girl is a boy. It's also possible that she's just a runt. As long as she's eating those small rats weekly and defecating normally (no nasty runny poo), I wouldn't be concerned at all about her weight. Just to clarify, a "small" rat is 50-80g. I would never give an 800g BP a medium rat.

You can also post a few pics just so we can see her body condition.