More party member mean more of everything: the world will have to be bigger, the town will have to have more population, the battles will have to involve more enemies... all of this means higher processing requirements, and if Obsidian is planning to make the game accessable to people with less powerful computer systems, it would mean the graphical quality of the game would have to be decreased.
And before anyone says that graphics don't matter: I'm not talking about decreasing the texture resolution or smoke effects, I'm talking about less different models, less viewing distance and less animations, and that's not something I would like to see omitted.
3-4 party members, main character included, are enough in my books. I'm more of a quality over quantity sort of guy anyway.