Jupp, smoking does make you thinner. Especially noticeable with women. It has the effect of giving a kick of energy and suppressing hunger.
Weight means nothing, take a look at BMI. You don't look that twiggy to me (said the thin lanky fencepost) but as somebody said America is a land of fatties. In Europe we'd be normal to bulky.
As for the coworkers I think you should confront them about how they suppress their sorrows with food and detail al the negative health consequences with it. If you're the passive-aggressive type it would serve you well to say something before blowing up (never done that myself, nosir
)