A horse racing glossary was printed in the column “The Sport Tide” by Flem R. Hall in the Fort Worth (TX) Star-Telegram on October 15, 1933:
     
“BIG APPLE—The major tracks of the country.”
   
An article in the Lubbock (TX) Avalanche-Journal on May 20, 1934 was slightly different:
   
“Big Apple—the major tracks of the country such as Belmont, Arlington Downs, Churchill Downs, etc.”
   
 
Newspapers.com
15 October 1933, Fort Worth (TX) Star-Telegram, “The Sport Tide” by Flem R. Hall, pt. 2, pg. 1, col. 6:
FRYING PAN CIRCUIT—The small half-mile tracks of the “sticks,” or rural districts. Also known as the Leaky Roof Circuit and the “bull-rings.”
BIG APPLE—The major tracks of the country.
   
Newspapers.com
20 May 1934, The Sunday Avalanche-Journal Lubbock (TX), pg. 5, col. 3:
Horse Racing Coming To Panhandle In
“Big Style” Next Month When Amarillo
Plays Host To Nine-Day Racing Meet
AMARILLO, May 19. (Special)—Horse racing is coming to the Panhandle in big style next month, and those who wish to learn about ponies should start now. The Panhandle race meet at Panhandle on June 7-10 an the nine-day spring meeting of the Tri-State Fair and Racing association in Amarillo June 15-25 will bring to this section all the thrills found on the “big apples.”
 
Do you know what the “big apple” is? If you don’t you might as well learn now what it and a lot of other expressions peculiar to the race track mean. You’ll be hearing a lot of funny expressions at the races here, and if you’ll study the definitions below, you will be in a position to know what they mean.
 
Big Apple—the major tracks of the country such as Belmont, Arlington Downs, Churchill Downs, etc.
 
“Frying Pan Circuit”
Frying Pan circuit—the smaller tracks where most the thoroughbreds start their careers.