I believe it has to due with insulin use, the exact mechanism is debatable, but I think it has something to do with resistance to insulin which causes the distension. You can see this occur in older males, I see a lot of them in the gym who are ripped to shreds, super lean, veins popping out, cuts in the arms and chest, but with a massive gut. These guys aren't heavy abusers and probably never touched insulin in their life (just older male gym rat types who juice but don't touch the heavy shit) and have massive pregnant sized guts.
It's been a while since I really cared to bother looking into this stuff, but I think there's a connection in the same mechanism. As the bb'r uses more and more insulin you become more and more resistant to it and the liver slows down natural production causing more storage of fat deposits in the abdomen. There could be other issues such as IGF production (from GH use) that causes as well, or some synergistic effect of the slin and GH together, but I think it's more of a slin issue as GH production slows down rapidly in men as they age and I still see massive guts on older guys.
Of course I could be totally wrong, but I think there's some correlation there, just not sure what the exact cause is.