That trend is reversing. The top young prospects are traditional baseball builds again.
Another reason the old guys looked smaller is that today they use tiny bats. In the old days, most used the heaviest bats they could swing.
Not too long ago I saw some footage of the 1968 World Series (Tigers v. Cardinals) and was really surprised how slender the players looked.
So I checked some of the player weights in the Baseball Encyclopedia, and concluded that most of the guys back then were well under 200. The big guys like Norm Cash were 195-200.