Well, the number of people claiming Taylor Swift, a person who had an eating disorder, isn't skinny is certainly proving the OP's point. That being said, I think part of this is that the person is using a simplified, slightly cartoon-ish art style, which communicates reality through minimal lines. Our brains interpret reality through exaggerations in drawings like this; if they were to communicate youth in this style, they would probably make the figure's eyes 30-40% bigger than they would be in reality. I think that, the more detail they added, and the closer to photo-realism they got, the less, "plus-sized," she would appear.

Another aspect is the angle of the photo; it's a somewhat candid photo taken while she's walking. She looks significantly thinner in the photo below, where she's posing at a more flattering angle for the camera.

None of this is to invalidate the OP's point, but to offer an explanation for this mentality. If your exposure to women is mostly anime characters drawn with exaggerated proportions and Instagram models posing at optimal angles, you can convince yourself that Taylor Swift isn't skinny by seeing a simplified sketch of her.