It all depends on the bird's build, and there is no general "your bird must be this weight to be healthy." In the wild, the birds typically weight between 70 and 80g, it's only in captivity that we have started breeding for the bigger birds. And even now, a lot of breeders are moving away from the big birds because it's not normal for a cockatiel to be that large. As long as she's eating good you should be fine. Have you ever heard of a keelbone check?