If your DVD has the extras on it, you should watch them.
Basically, they wanted to make an entertaining story. For that they needed rivalry, a hero and a villain. Originally, Ferrigno was to be the villain. The problem was that he is just too nice, despite being like eight foot tall and half a tonne, he just wasn't a plausible villain. Likewise, Arnie had never messed around with Franco Columbu's head - they had a business together, after all.
So they turned it around and made Arnie the villain.
If you've not got the extras on your copy, some of them are on youtube.
It's a good insight into the training and the whole atmosphere of the time, but it's a movie, not a documentary