Interesting. My amateur [brewing] mind would look at the SG of a beer and imply that extra 'stuff' put in contributes to the SG, mainly sugars. That implies to me that a low SG would have less sugar remaining, and hence would contain less energy that a person might convert to their spare tyre. Is this correct?
I've also heard (and I hear a lot of things) that it's the presence of alcohol that contributes to weight gain. By what mechanism, I'm not sure. My wife's gym instructer said something about how it tricks the body to storing things as fats. Hard to know what to believe.