No expert on drugs or anything, but IMO there is definitely a difference. Since somewhere mid 2000s most physiques starts looking softer and less conditioned. Only a handful of guys really looks shredded now.
Mr Olympia '06 '07 '08 '09 '10 '11 '12 all seem to have this phenomenon.
Like you say, when they stand relaxed they looks pretty smooth all of them, and things get better when they pose.
Earlier ronnie ca 98-2002 shows - they look better relaxed.
Also a difference going back to the seventies/eighties they looked different again.
Must be some trend in drugs that causes this...
Maybe I'm out of my depth here, I haven't slept well for two days (the shows are broadcasted 4.00 am in sweden)