Sure there is. Look at Detroit's new stadium, or Shea, or Pac Bell. They might not have Polo Ground's-esque dimensions, but there definitely not band boxes.
And if you look back at some of the older stadiums, there were just as many tiny parks then as now.
Before expansion in the late-80's, 49 HRs by McGwire in 87 seemed shocking. I remember reading articles claiming that 50 would never be topped. Adding the Marlins, Rockies, D-Rays and D-Backs added 45 pitchers to rosters that should have never been there. Post-expansion is when the HR records have all been traditionally broken. I believe that was the case in 1961 as well, and is the case now.
As far as steroid use, who knows. Of course prior to 1990, there really was no weight training to speak of either way.
And Coors Field doesn't count. You or I could hit one out there. :)