A lot of medical professionals (and, actually, Yaz' own website) explain the effect like "tricks your body into thinking it's pregnant." I don't think that's the most accurate way of putting it, but it's certainly the most common explanation.
I don't know of any research on HBC and appetite. I do know that I have an increased appetite on HBC, and gained weight accordingly (about 5 lbs).
However, there are studies showing that birth control pills are not linked (http://www.vaginapagina.com/index.php?title=Hormonal_Birth_Control#What_about_weight_gain_as_a_side_effect.3F) to a statistically significant weight gain, so if I had to make a non-educated guess I'd say that this effect doesn't happen to everyone, has a lot to do with what your appetite was like before (I'm an extremely small eater naturally), and thus produces some gain for some people because of that, but not widespread enough to be statistically relevant.
no subject
I don't know of any research on HBC and appetite. I do know that I have an increased appetite on HBC, and gained weight accordingly (about 5 lbs).
However, there are studies showing that birth control pills are not linked (http://www.vaginapagina.com/index.php?title=Hormonal_Birth_Control#What_about_weight_gain_as_a_side_effect.3F) to a statistically significant weight gain, so if I had to make a non-educated guess I'd say that this effect doesn't happen to everyone, has a lot to do with what your appetite was like before (I'm an extremely small eater naturally), and thus produces some gain for some people because of that, but not widespread enough to be statistically relevant.