But you were right about hitters being bigger, stronger and in better shape than in years past. That seems to be true all across sports. Look at offensive linemen in football. It was kinda rare to have 300 pound guys, now every player is that big. Or basketball, 7 footers weren't as common as today.
And this doesn't even take into account video and scouting. People who play sports now have a year long dedication to it. In years past, players had to have an off-season job. It really is a much different game in modern times. And, IMO, the use of relief pitchers is the biggest difference in baseball.