Sure they scale down. Her ability to win in her homestate has nothing to do with her ability to win anything more than those 6.95 million votes locked up in California.
How are these Arnold and Bush voters "locked up" by Boxer?
I think you have confused local politics quite severely with national politics. Boxer got 57%; how much did Kerry get, something like 54% (wild guess)? That leaves 3%. Ooh, big gains that Boxer made with the Nader voters, huh?
Quit acting like Boxer getting the most votes for a Senate race means anything. What if the largest state were Republican leaning and the winning Senator-elect won the most votes in history? Would you be touting that fact? No, because it shouldn't matter. California's a big state, naturally lots of votes will be present.
If you seem to be thinking that Boxer has tapped some big base of Republican voters and would do well nationally, you're terribly mistaken.
In regards to Arnold, maybe it didn't occur to you that Arnold isn't exactly conservative. He's pro-abortion and pro-gay rights. That sort of thing plays well in California.