Remember when a serious homerun hitter was a guy who chipped in 25 or 30 per year? I do. I remember when George Foster hit 50 for the Reds, and we thought that was a mind-boggling number. All those years that Mike Schmidt led the NL, he never had much more than 40, often less. Today that's a sub-par year for Bonds or Sosa.
Many ballparks were just flat out difficult to homer in unless you were a dead pull hitter. Center field fences are now typically just over 400 ft from home plate. OTOH, the old Braves ballpark used to be called the launching pad during Aaron's heyday. Wrigley was another easy one if the wind was blowing out. But these were the exceptions.
I remember the year Foster hit 50 (1977?). If I am not mistaken he was the first to hit that many in over 10 years and no one did it again for at least 10 years after that. You're right that now 25-30 HRs is not considered outstanding, meanwhile how many times have you seen a batter totaly screw up an attempt to bunt?