“I’m not fat—I’m cultivating mass”

“I’m not fat—I’m cultivating mass” is a jocular saying that has been printed on many T-shirts. The line was spoken on the American television sitcom It’s Always Sunny in Philadelphia by Ronald “Mac” McDonald (actor Rob McElhenney) on an episode that aired on September 15, 2011.
 
Similar sayings include “I’m not fat—just short for my weight,”  “I’m not overweight—I’m undertall,” “I’m not fat—I’m flooded,” “I’m not fat—I’m fluffy,” “I’m not fat—I’m insulated,” “I’m not fat—I’m just big boned,” “I’m not fat—I’m just well-marbled,” “I’m not fat—I’m just easy to see,” “I’m not fat—I’m just so sexy, it overflows” and “I’m not fat—my stomach is just in 3D.”
     
 
Wikia: It’s Always Sunny in Philadelphia
Frank’s Pretty Woman
Season Seven, Episode One
Writer Scott Marder & Rob Rosell
Director Matt Shakman
Air Date September 15, 2011
“Frank’s Pretty Woman” is the first episode of the seventh season of It’s Always Sunny in Philadelphia.
 
Frank wants to marry a prostitute and the gang can’t talk him out of it. While Dee sets out to help the hooker with the second act of her life, Charlie takes a different tack: Devising an elaborate scheme to set Frank up with another woman, one who will like Frank for who he is. Meanwhile, Mac has put on 60 pounds of weight. “I’m cultivating mass,” he tells Dennis.
 
Twitter 
Patrick Thomas
‏@patrickcthomas
I’m not fat. I’m cultivating mass @jaredkeesee
9:15 PM - 15 Sep 2011
 
Twitter
Roberto Febus
‏@Rattlesnake_Rob
“I’m not fat, I’m cultivating mass” hahah #always_sunny
9:19 PM - 15 Sep 2011
 
Twitter
Dotcomaphobe
‏@Dotcomaphobe
Like Mac on “Always Sunny,” I’m not fat. I’m just “cultivating mass.”
8:53 PM - 16 Sep 2011
 
Twitter
London
‏@LondonFATS
I’m not fat, I’m cultivating mass
2:51 AM - 11 Oct 2011
     
Mandatory
THE 100 GREATEST QUOTES FROM ‘IT’S ALWAYS SUNNY IN PHILADELPHIA’
By Cory Dudak Feb 28, 2013
It’s hard to believe that the hysterical FX series “It’s Always Sunny in Philadelphia” has been on the air for eight seasons, with production of season nine underway.
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47. I’m not fat. I’m cultivating mass. -Fat Mac