Much easier to tell what kind of shape she's in with a picture. I wouldn't be too concerned so long as she has good muscle tone though. Plenty of folks, myself included, grow carpets on the slower side. They can also sometimes be tricky to switch over to rats, another reason she might be on the smaller side if the previous owner was showing some tough love to a stubborn converter. Many of the aussie species don't handle overfeeding well and it's better to have a slightly skinny animal as opposed to a sausage.