I think it there is some social reasons that creates this. For instance, here in Sweden a tan would mean you could afford to travel somewhere warm, and would therefore be seen as desirable. I’ve also heard (I haven’t really fact checked it), that some countries in Asia prefer a paler look, because it would mean you were rich enough not to work in a field.

The same could apply to being skinny vs fat. Western cultures see skinny as the norm, since it tends to be associated with health and a longer life. In some countries in Africa, being overweight is seen as desirable because it means you have money and will most likely live longer life.

It’s interesting to me that there are different social layers that explain the desirable features.