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This is Arnold Schwarzenegger at his contest-biggest, on stage at the 1974 Mr. Olympia Contest at New York’s Madison Square Garden. There is a distinct difference in the way Schwarzenegger shaped his chest for this contest and how he prepared for the 1980 Mr. Olympia Contest. Arnold seemed to have concentrated more on training the clavicular portion of his pecs in 1980 as evidenced by the different look that year. Arnold’s pecs appeared less round or “boobish” in 1980. Maybe he was influenced by Zane who had won the three years previous to 1980. Zane trained his to make his upper outer pecs stand out more and the lower part fall off into shadow under the stage lights. Below is a photo of Arnold around the time of the 1980 Mr.Olympia. Have you seen a photo of him with his pecs structured so upper chest dominant prior to 1980? He has more of an athletic look than a hanging pec bodybuilder look.
Below: Pre 1980
Without a doubt, Schwarzenegger was a hyper responder to drugs..made a huge difference in his appearance once on them. My point stands; higher bodyweight affects chest size.
IMO the difference has nothing to do with training. It’s because of the larger differential in body weight in the two shows. Whenever anyone loses weight some areas go down, and in his case his triceps and thighs were two. Chest just lost a lot of volume with the loss in weight.
It has to do with his body’s response to the drugs he took. His pecs blew up into balloons, his most dominant body part.