“What do you call a fat jack-o’-lantern?”/“A plumpkin.”

A popular Halloween riddle about pumpkins and jack-o’-lanterns is:
 
Q: What do you call a fat jack-o’-lantern?
A: A plumpkin.
 
The joke (also in a “fat pumpkin” version) has been cited in print since at least 1989. The term “plumpkin” for a fat pumpkin had been used in print infrequently in the 1960s and 1970s.
 
   
14 October 1965, The Evening Republican (Columbus, IN), pg. 17, col. 4 photo caption:
PLUMP-KIN—As hard as he tries, Randal Black, 4, can’t get his arms around this huge, prize-winning pumpkin at the Michigan State Fair.
 
27 October 1976, Colorado Springs (CO) Gazette Telegraph, pg. 3-F, full page ad:
PLUMPKIN PIE
(Sooper supermarket.—ed.)
 
14 October 1977, The Sun-Telegram (San Bernardino, CA), pg. A-2, col. 1:
Plumpkin
Kelly Rieman, 5, of Royal Oak, Mich., is ready for Halloween. She grew the 162-pound pumpkin from a 39-cent packet of seeds.
   
Google Books
Oodles of Riddles
By Lori Miller Fox
Pictures by Sandy Hoffman
New York, NY: Sterling Publishing COmpany, Inc.
1989
Pg. 45:
What do you call a chubby jack-o’-lantern?
A plumpkin.
   
Google Books
Awesome Jokes
By Charles Keller
Illustrated by Jeff Sinclair
New York, NY: Sterling Publishing Company, Inc.
1997
Pg. 87:
What do you call a chubby jack-o’-lantern?
A plumpkin.
 
Google Groups: uk.local.devon
Halloween riddles….print for your kids
Jon Down
10/1/01
(...)
What do you call a fat Jack-O-Lantern?
A plumpkin
 
26 October 2004, Statesman Journal (Salem, OR), “Getting into the spirit of Halloween,” pg. 1E, col. 5:
Q: What do you call a fat jack-o’lantern?
A: A plumpkin.
   
17 August 2005, Los Angeles (CA) Times, “The Kids’ Reading Room: Jokes & Riddles,” pg. E15:
What do you call a huge pumpkin? A plumpkin.
Drew, 7
Grant Elementary
Santa Monica
   
Twitter
gaye Dimmick
‏@gdimmick2000
What do you call a fat Jack-o-lantern? A Plumpkin!
7:43 PM - 19 Oct 2009
 
Google Books
Jokelopedia:
The Biggest, Best, Silliest, Dumbest Joke Book Ever!
Compiled by Eva Blank, Alison Benjamin, Rosanne Green, Ilana Weitzman and Lisa Sparks
New York, NY: Workman Publishing
2013
Pg. ?:
What do you call an overweight pumpkin?
A plumpkin.
 
Google Books
The Super, Epic, Mega Joke Book for Kids
By Whee Winn
Grand Rapids, MI: Zonderkidz
2016
Pg. ?:
What do you call a fat pumpkin?
A plumpkin.