Hmm, I definitely agree that Dreamworks films tend to be about outcasts and ‘rags-to-riches’ style storylines, but I’d hesitate to say that Pixar protagonists are universally the leaders. Here are the exceptions:
A Bug’s Life: Flik is obviously not a leader, and this is probably the most most conventional rags-to-riches plot out of all the Pixar films.
Finding Nemo: I’d say that Marlin is as much of a social outcast (largely self-inflicted) as any Dreamworks protagonist, and is even outcast from his own son on many levels.
The Incredibles: This probably depends on whether or not you think the movie really starts with Bob as a down-on-his luck insurance guy, which is where I personally think it starts. The stuff before that is a prologue, in my opinion.
Cars: Lightning McQueen is not on top of the world at the start. He is very close, and he’s aiming for it, but he’s not there yet. The whole film seems to be a subversion of the rags-to-riches plotline. At no point does McQueen achieve conventional success, but he gains more important things along the way.
Ratatouille: Yeah, you don’t get much lower than a rat.
WALL-E: Except possibly a garbage man. No, not just that, a robot garbage man doing a completely meaningless task, alone, for which he has no expectation of any reward or gratitude. At least Remy had a job with the slightest hint of purpose by smelling for poison in the rats’ food. WALL-E is even lower than that. He is the ultimate woobie (look it up on TV Tropes).
Up: An old man whose house is in perpetual danger of being demolished by the big evil corporation of doom is hardly on top of the world.
I certainly agree that Woody and Sully start off in the best possible positions, and there are arguments for Mr Incredible as well, but Pixar is no stranger to the underdog.