Manhattan developers are building increasingly large condos, New York City's upscale answer to the suburban McMansion.

At the peak of the Manhattan real-estate boom in 2007, the average new condo—from studios to penthouses—was 1,265 square feet. Now, new condos average 1,564 square feet, a 24% increase, said Kelly Kennedy Mack, president of Corcoran Sunshine Marketing Group.

The...