well they are eating nutrient dense foods, that would require less digestive apparatus. On the other hand cows eat massive quantities of nutrient impoverished grass so they need a big digestive machine to cull the quality sustenance.
I have a feeling that bulking bodybuilders are closer to cows then lance in terms of their food's nutrient density. Perhaps thats why BBers guts tend towards the bovine
many pro bbers from the seventies would gorge themselves on massive amounts of food off-season to bulk up, then lean down for a comp with a trim waist.
hell, serge nubret would eat 6lbs of meat in one meal, but had a very small waist.
oliva, buchanan, etc same.
of course, todays bbers have a shit load more muscle on their bodies and, whether people understand this or not, when you lift heavy ass weights (ronnieism) in rows, deads, squats, etc you are going to develop some thick abs, obliques, erectors, etc.
i don't know exactly what's causing it but i don't buy the bulking theory at all. too much evidence to refute it.