I think the idea is that Darcy being richer would live in the square instead of on the adjacent street. The houses in the square were generally bigger, but not on a par with Park Lane or Upper Brook Street, which were very posh (according to what I've read).
Although outside of Mayfair, Portman Square was still pretty nice (a duke lived there, after all), and within walking distance of the park.
Although outside of Mayfair, Portman Square was still pretty nice (a duke lived there, after all), and within walking distance of the park.