I think its just denser muscle, i.e. its not marbled in so much fat. You look at big beefy guys in gyms now - and they are big, but they look puffy and not very well defined. They almost take on a 'round' look.
Once they stop training they deflate sort of like a balloon - I think its more to do with the fact they stop consuming so much copious protein, than stopping training.
I was out at the Wests Tigers v Bulldogs NRL match yesterday - and some of the players are massive. Then I noticed a bulldogs player only retired for 2 years working as a trainer. He looks about 15-20kg lighter than his playing weight just over 2 years ago. And then all the old footballers working as administrators, support etc... are all much tinyier than they once were many years ago.
There may be something to do with isolated weight training, but I think when you are working all your muscle groups through bodyweight/isometric training you get a more natural look - combined with right nutrition - the ripped factor is there. And since its very hard to lose muscle unless you've been fasting for a significant period - then over time you will not start 'deteriorating' away like most muscle heads do - I think they just loose some of the water and fat weight from the muscles.
That being said over a significant amount of time if you don't use it you will lose it. So if you stop physical training altogether your body will say - hey we don't need all this muscle, time to conserve our resources and the muscle slowly gets eaten away. I have no plans on reducing my training well into old age so that doesn't concern me. Doug at 56 is rock hard.
All this is speculation on my part too
