It's a matter of talent. On their first album, the disparity between the pair in terms of ability was noticeable to the point of being uncomfortable. It was clear that, of the two, the guy who called himself "Dre" at the time was the star of the show. Not that Big Boi was bad; he was just a lot less good than his partner and had a way sillier name (still true).
A whole lot of things changed within the Outkast universe between their first and second albums, though. For one thing, it was obvious that Big Boi had done a shit-ton of rap pushups or sold his soul to the devil or something, because his talent as a rapper increased dramatically, so much so that definitively identifying one as better than the other was almost impossible.
Meanwhile, as stated earlier, "Dre" went from wearing Atlanta Braves jerseys and rapping about drugs to wearing turbans and explaining how the New World Order would destroy us all once the year 2000 rolled around.
With that in mind, here's a question: Is it possible that one of those changes led to the other? The group had been together for at least four years by that point, and there's a good chance Andre Benjamin had been considered the "star" of the group the entire time based on nothing more than obviously being the superior talent. That's not the kind of status an entertainer relinquishes without a fight. For whatever reason, the weapon Andre 3000 decided to bring to that fight was football shoulder pads and a pair of furry pants.
"Please love me more."