This is like saying 2 = 3 because 2 != 4.
This yellow firetruck is red because it is not blue.
The aircraft is grounded because it is not flying at above 20k feet.
Etc.
Skinny literally means 'thin or very thin'.
It does not mean 'not plus size'.
Look up any English dictionary's definition.
They way OP from the image used the word was nonsensical.
The only thing that is 'literally skinny' is the drawn-as-skinny figure, based on the picture of Swift, who is herself not skinny.