You are right. Arnold can't match Colemans bloated GH stomach. Arnold was silly, for some reason he preferred to have a tight, sharply defined midsection.
Bodybuilders today are fooling themselves if they think Coleman and all the other blocky blobs are an improvement over Arnold.
Arnold had large hips and massive obliques in most contests. Considering he was maybe the biggest drug abuser of his generation, their is no doubt he would have had the same mentality if he competed 30 years later. He was lucky to have a great line at his time as drugs couldn't allow his gut and his waist growing. But the fact is that if he had competed during Coleman's area he would have been more predisposed to destroy his line than Ronnie himself who started his career with a small waist and large clavicles. Add the fact he had more weak body-parts like shoulders or legs, he would have had a much more unbalanced physique than you think.
Arnold was just lucky to compete during a period where drugs only sublimate your physique without adding bad visible effects like gh gut, balonie etc... but I'm sure his career would have been totally different with a current drug protocol and against more genetically gifted guys than him...
Remember that, the first goal of every people on the world who train is gaining muscle. The main objective of the bodybuilder is to be bigger than yesterday and skinnier than tomorrow, that's why you pump your muscles everyday. This is how Arnold was thinking in his time and this how he would have thought if he would have competed today.