“I’m not overweight, I’m just undertall” is a jocular dieting excuse, printed on gift items such as T-shirts. The saying was popularized by the Garfield syndicated newspaper comic strip, created by Jim Davis. The cat, named Garfield, is fat and loves to eat.
 
In the Garfield strip on September 21, 1979, the cat says: “THAT MEANS IF YOU’RE UNDER 6’4” YOU’RE NOT OVERWEIGHT, YOU’RE UNDERTALL.”
 
The similar saying of “I’m not fat—just short for my weight” has been cited in print since at least 1952 and was popularized in the 1960s by comedienne Totie Fields. Other similar sayings include “I’m not fat—I’m just easy to see,” “I’m not fat—I’m just big boned,” “I’m not fat—I’m just well-marbled,” “I’m not fat—I’m fluffy,” “I’m not fat—I’m cultivating mass” and “I’m not fat—I’m just so sexy, it overflows.”
   
     
Wikipedia: Garfield (character)
Garfield is a fictional character and the title protagonist from the comic strip Garfield created by Jim Davis.
 
Character
Fictional biography
Garfield was born June 19, 1978, in the kitchen of Mamma Leoni’s Italian Restaurant and loved Lasagna the day he was born. Ever since then, it has always been his favorite food. According to his grandfather, he was born five pounds, six ounces (He was out of town at the time, and when Garfield asks how, his grandfather said he heard the scream), surprisingly enough, he managed to fit in a tiny bed. Later in his life, Garfield accidentally runs across his mother again one Christmas Eve, and meets his other grandfather for the first time. In June 18, 1978, when Jon came to the store, he had to choose between Garfield, an iguana, and a pet rock. In his cartoon appearances, Garfield usually causes mischief in every episode.
 
Urban Dictionary
undertall
A condition caused by the height and weight ratio not working out just right.
She’s not overweight, she’s undertall.
by cowboy_up Sep 4, 2009
 
21 September 1979, Boston (MA) Globe, “Garfield” (comic strip) by Jim Davis, pg. 32:
HERE’S THE NATIONAL FAT WEEK “WEIGHT-HEIGHT CHART”
ACCORDING TO THIS, IF YOU WEIGHT 200 POUNDS, YOU SHOULD BE 6’4”
THAT MEANS IF YOU’RE UNDER 6’4” YOU’RE NOT OVERWEIGHT, YOU’RE UNDERTALL
(The comic strip is online here—ed.)
   
Google Books
The Grass Is Always Greener Over the Septic Tank
By Erma Bombeck
New York, NY: Fawcett Crest
1982, ©1976
Pg. 125:
Women weren’t overweight. They were undertall.
 
20 March 1982, Hutchinson (KS) News, “Give Garfield a chance” by Kyle Hopkins, pg. 4, col. 5:
Garfield is like no other comic strip character. He is overweight, lazy, and extremely cruel. The first trait I mentioned, overweight, is a poor choice of words. Actually he is undertall.
   
Google News Archive
14 June 1982, St. Petersburg (FL) Times, “Off/Beat” by Jan Glidewell, Hernando Times, pg. 1, col. 2:
“But I’m not overweight. Just under-tall.”
 
22 November 1982, Rockford (IL) Register Star, “Comic-strip cat claws his way to lofty perch on best-seller list” by Timothy Harper (The Associated Press), pg. 1B, cols. 3-4:
He said he draws each strip about four weeks before readers see it. So he can tell fans that another diet is coming up for the cat who complains that he’s not overweight—he’s undertall.
 
Google News Archive
27 December 1984, New Straits Times (Malaysia), “I’ve decided it’s time that I, Garfield, graciously paid back my many fans for their loyalty. So, here’s the true story behind me and my sidekick, Jim Davis—how lucky can you be?” by Maria Niesley, Times Two, pg. 6, col. 3:
“I’m not overweight. I’m under-tall.”
 
10 March 1986, Toronto (Ontario) Star, “Excuses are blooming in T-shirts these days” by George Gamester, pg. A2:
I’M NOT FAT, claims one from Bruce Bryer of West Hill, JUST SHORT FOR MY WEIGHT. And I’M NOT OVERWEIGHT, explains another from George Karpati of Toronto, JUST UNDERTALL.
 
31 March 1999, The Advocate (Baton Rouge, LA), “Saturday’s Fat Boy 5K gets national attention”:
By the way, the slogan for this year’s run is “I’m not overweight. I’m under tall.”
   
OCLC WorldCat record
Difficult aneurysms for endovascular treatment: Overwide or undertall?
Author: Brinjikji W.; Cloft H.J.; Kallmes D.F.
Edition/Format:  Article : English
Publication: American Journal of Neuroradiology, v30 n8 (2009 09 01): 1513-1517
Database: Copyright 2012 Elsevier B.V. All rights reserved
Other Databases: MEDLINE; British Library Serials; ArticleFirst