I just wanted you to know I can relate to this. Only it is my twin sister who can eat what ever she wants and not gain an ounce. I weigh about twice as much as her. But she doesn't exersice, she smokes, and she doesn't get yearly well women check ups. I learned in February that at 49 years old and peri-menapausal, that I have ostioporosis, my sister has yet to be checked for it. In some ways I take better care of myself.
You may be taking better care of yourself than your co-worker, even if she is thinner.