Smoking and sedentary means fat?

Today in tutorial we considered the case of a post-menopausal woman and her risk for osteoporosis. The case described the woman as a sedentary smoker, but made no mention of her weight. The tutor then made a comment along the lines of "And is she also obese? Probably!" I responded, "Probably not, because obesity is protective for osteoporosis and this woman already has low bone density." I don't even know what the association was in this case, it seemed totally random. Is it simply because people assume that sedentary = fat?

Other than that, the gynecology resident who taught the session was an excellent tutor. He explained the material well, shared career advice, and was generally friendly and helpful. I'm adding this bit because I don't want to generally complain about this tutor -- he wasn't bad. The stereotypes about fat patients seem to hold for just about every physician I meet through school, no matter what they are like otherwise.