Rob McElhenney told The AV Club in 2011 that he was inspired to make Mac gain 50 pounds for Season 7 of "It's Always Sunny" because of shows like "Friends" and "The Big Bang Theory." The actor and show creator noticed that as the characters got older, the ensemble cast members had more makeup on and looked better. "If anything, when you're in your late 20s, early 30s, and then mid-30s, you're getting less attractive," he joked.
McElhenney decided to write an arc where Mac believed he'd "gained mass" and now looked truly intimidating but, ever in denial, he'd merely put on an unhealthy amount of weight. The intent wasn't to make fun of overweight people but to mock Mac's absurd body image: "It was more about the character specifically."
To gain the weight for the episodes, McElhenney had to eat five 1,000 calorie meals a day. At first, this meant proteins like chicken breasts and vegetables. But as he confessed to actor Nick Kroll, he eventually started eating Big Macs and donuts instead (via The Wrap). It was a fun diet apparently, as he said, "I never felt lethargic. I felt great."
McElhenney went on a diet and was back to his standard size by Season 8, but he wasn't done using his body as a comedy prop. The next effort would be much more difficult however.
