I also wonder if it has something to do with the diets of today vs 20-30 years ago.
I'm not 100% sure but I think the bodybuilders back in the 70's and 80's era were pretty much lean year round. You could always see them with full 8 pack abs and a pretty tight waist. Maybe it's the off season big bulkups and gaining massive amounts of weight in a short time that stretches the stomachs out whereas the oldschool guys would just slim down a bit and dry out for the shows instead of getting monstrously huge and then cutting way down?
I have been researching this all afternoon and looks like nobody really has a solid answer.
I did see one guy, Emeric D (ifbb pro) state in black and white "gh and insulin in high doses" and he approved someone else's post that insulin stores fat. The answer?