If you are going to compare Rocky with todays big heavies that's fine, but keep in mind that if Rocky were born in 1980 he probably would have weighed more than 187lbs... Also remember that size has drawbacks, and don't forget the law of diminishing returns. Big only gets you so far and slows you down while sucking energy. In contrast, A 160lb man like golovkin has plenty of power to starch anyone, now imagine a 200lb Golovkin and you now have someone that resembles what a modern day marciano would be like. Put that guy in any heavyweights chest and see what happens!
Yeah, that fight may be a bit overrated in terms of action, but it's extremely underrated in terms of skill displayed by both competitors.