Question about my rats size?

I'm not able to view your picture, but I could make a guess. I've seen that happen with rats with crooked or broken teeth. They have trouble eating and therefor don't get enough food to grow. Check her teeth to see if they look straight or overly long. If that isn't the problem, you may want to take her to the vet. Some size difference is normal, but if they're as different as you say, she might have something wrong with her.