I had read before the film was released that they went with someone of Topher Grace’s stature because they wanted someone who would basically be Peter Parker’s doppelganger (i.e. not too big). I think they wanted to keep him smaller so they wouldn’t have to explain: 1. How the suit bulked up Brock so much when it didn’t do that to Peter or 2. Why Lou Ferrigno was working as a freelance photographer. Either way it’s a classic example of Hollywood dumbing down something for the masses that the real fans would just accept.