Those who hit 40 or more home runs during the 1920s either began their careers in that decade (Lou Gehrig, Mel Ott, Chuck Klein) or reached their peak then (Babe Ruth, Rogers Hornsby, Cy Williams). If it was the ball that was responsible for the big surge in home runs, then the old and the new batting stars alike would have seen dramatic increases in homers. But that was not what happened.
When Roger Maris broke Ruth's home run record in 1961, it was during the first year of baseball's expansion beyond the 16-team limit that had existed since the beginning of the century. With expansion teams stretching the pitching thin, many batters had banner years. But the three top pitchers all had earned run averages under 3.00 in 1961, while throwing the same ball as the rookie pitchers who were rushed into the big leagues and the washed up pitchers who were able to hang on with expansion teams.
The most recent escalation of home run hitting has come at a time of bigger players and smaller ball parks. Not only have the new stadiums been built with shorter distances to the fences, older parks like Yankee Stadium have been remodeled to bring the fences closer. It used to be 415 feet to the left field bullpen in Yankee Stadium, but it is not that far to dead centerfield in most of the major league parks today. None has the 461 feet to the centerfield wall that Yankee Stadium had during the careers of Ruth, Gehrig, DiMaggio and Mantle.
You can never prove a negative, so those who want to believe that the ball has been juiced can continue to believe that. But the evidence is against them.