Im not arguing specifically about that one girl, she probabably is a little too thin, but really not by much, ideal BMI is different for everyone, and depends a lot on your body type, bone structure, how much muscle you have is especially a big one. and other things specific to an individual, like metabolism, some people do better with low fat diets, some better with higher fat diets, etc.. some people naturally have less fat than others, and good diets definitely reduce the look of fat. when you are eating good, healthy fats, not the processed greasy bad fats most people get.  my BMI is actually pretty normal for the healthy range, but if you were to look at me, you would think it were much lower.
really though, in this pic, she just looks lean and somewhat athletic, not saying shes ideal, but for someone with a thin, skinny hip body type, she really looks pretty good.  nothing like that 85lb skelaton you posted. but im not going to argue heights and weight because we just dont know, but visually this girls got a bit more muscle. and looks like she actually eats. i wish i had a better picture, but from the little i can see it just seems like shes got a skinny body type. 
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so just for fun, even though i disagree, lets say this girls 10-15lbs under her ideal, healthiest weight. is that worse than being 10-15lbs overweight, any nutritionist ive talked to would disagree. being a little bit under is in most cases quite a bit healthier than being a little bit over.
really thought, this specific girl wasnt my point, its our cultures mentality in general that being overweight is ok, even though its unhealthy, extremely unhealthy in many situations, we cant point it out and give them shit for having a poor diet and lifestyle, like this kristie alley lady, all those adds with her braging about being a real woman and real women have curves etc.. our cultures reactions is "yeah you tell em!" but when we see a picture of someone whos even just slightly underweight, which is actually still healthy, its suddenly a bad thing to think thats attractive.  imagine if we had some famous actress, who was 25lbs underweight, rather than 50lbs overweight, in all these adds bragging about not eating much, and being skinny. people would flip shit. both are unhealthy, but its ok to encourage being fat for some stupid reason.  maybe its because being skinny takes a lot of effort? in modern times, maintaining a healthy diet and level of fitness is actually a shit ton of work.  its become the norm to be like 10-30lbs overweight, its pretty rare to see people who are actually at an ideal weight, and even more rare to see someone quite a bit under weight. maybe thats why.