I think it's more "chudy" http://dictionary.cambridge.org/dictionary/english-polish/skinny
szczupły is healthy thin, only about a person.
chudy is thin about many things, and skinny about things like food (with less fat). It's less positive than szczupły about a person.
To my ear it sounds like the difference between "as well" and "too". So basically the same, but kinda stylistically different.
I guess I can add "także" here. It makes more sense to me with "she's thin, I want to be thin as well" than with "I am rich, I want to be thin as well".