Butler and Gaines approached him at the last minute in BB terms, so he was down a little from the previous year.
Not true.....Butler and Gaines had the film in the pipeline for several years, and had shot about 10 minutes of footage the prior year (73,74??), and showed that to investors in new York who put in the $150K to make Pumping Iron. So it had been established for sometime the film was going to shoot.
But Arnold had also been trying to break into the mainstream movie business, with limited success. Arnold had been offered a part in the film "Stay Hungry" and it wrapped a few months before the Mr. Olympia-and Arnold needing as much exposure as he could get took the Stay Hungry role and dropped a few pounds for filming, knowing full well he was going to have a more limited time frame to prepare for the 75 Olympia.
So Arnold's lower bodyweight was a result of him preparing and doing "Stay Hungry", not the filming of Pumping Iron at the last minute.