While "Hubbell" is indeed his last name, "Big" is the nickname Carrie and others use to refer to him throughout the series.
The reason for the nickname is never explicitly stated, but it's likely a combination of his:
* Physical appearance: He's tall and imposing, hence the "Big."
* Personality: He's often seen as confident, powerful, and hard to get.
* Financial status: His wealth and high social standing could contribute to the "Big" image.
The nickname fits Mr. Big's aura and creates a sense of mystery around him, making him even more intriguing to Carrie and the audience.