Wait a minute, Carrie didn't break up with Aidan to go back to Big. She and Aidan were living together and planning to get married when she realized she wasn't ready for marriage....he was. Big wasn't really a romantic part of her life tehn, he was a friend and has remained just a freind so far...who knows how the show will end
To answer the question why is he called Big. Before Carrie met him she would see him around town having power lunches, rushing off in his limo, etc. She didn't know his name but knew he was someoen imporatnt and powerful in the business world. So, when talking to her freinds about the hot guy she kept seeing around town, she refered to him as Mr. Big. I think that when she finally met him, the writers and director decided it was more interesting never to reveal his name. So, the girls kept calling him Big asa nick name and when alone with him, Carrie never uses his name.
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