From Chapter 1:
...a pair of enormous eggs, identical in contour and separated only by a courtesy bay, jut out into the most domesticated body of salt water in the Western hemisphere, the great wet barnyard of Long Island Sound. they are not perfect ovals—like the egg in the Columbus story, they are both crushed flat at the contact end—but their physical resemblance must be a source of perpetual confusion to the gulls that fly overhead. To the wingless a more arresting phenomenon is their dissimilarity in every particular except shape and size.
The differences between the two masses of land are all those made by man--one is more fashionable, the other less so. One is wealthier than the other. This highlights that _The Great Gatsby_ is going to be a social novel, one concerned with distinctions made by people and their behavior. The tone of the paragraph is humorous--sets up Nick's ironic perspective. Finally, the mention of the Columbus story recalls the founding of America and the "fresh, green breast of a new world" that the last, elegiac pages of the narrative evoke.