The question I ask to answer the too fat/skinny question is something like this:
Would the person's physical stature be an obstacle in an activity with moderate physical activity/coordination, such as road racing, skydiving or downhill mountainbiking?
If the answer is yes, the person is too fat/skinny. Big doesn't necessarily mean fat. Slim doesn't mean skinny.
This girl would have a problem in all three of the examples. No muscle behind the excess weight, therefore it's useless.