Harper Bazaar published “The Boys on the Big Apple” by John McNulty (1895-1956) in May 1947, on pages 166-169, 235-236. The article is noteworthy for its expansive definition of racing’s “The Big Apple”:
     
“But that’s when they are on ‘The Big Apple,’ which is the jockeys’ name for the circuit which includes the big and fashionable tracks — Belmont, Saratoga, Hialeah, Santa Anita, and a few others.
     
   
Google Books
Harper’s Bazaar
May 1947
 
Pg. 166 photo caption:
THE LITTLE MEN IN THE BIG MONEY: ...
 
Pg. 167:
THE BOYS
ON
THE BIG APPLE
by John McNulty
(...)
The Shorter Oxford English Dictionary, thankfully, is not often encountered at the tracks, where prescribed reading includes merely the Daily Racing Form or the Morning Telegraph, Each is a 25-cent publication devoted to the news of the 16,000 or 17,000 race horses continually in training in this (Pg. 168—ed.) country, and to news of their owners and jockeys.
 
Pg. 235, col. 1:
THE BOYS ON THE BIG APPLE (by John McNulty—ed.) (Continued from page 168)
(...)
But that’s when they are on “The Big Apple,” which is the jockeys’ name for the circuit which includes the big and fashionable tracks — Belmont, Saratoga, Hialeah, Santa Anita, and a few others. The small and more obscure tracks are called “The Merry-Go-Rounds,” and theirs is “The Leaky-Roof Circuit.”
(...)
The riders on “The Big Apple” are, as a rule, free with their money, fond of fine clothes, sleek automobiles and amply designed women. Maybe the law of compensation comes in here somewhere, for it is noticeable that in their hours of ease, the tiny jockeys choose tall, full-fashioned women for their playmates, and often for their brides.