By now, fans of 'Its Always Sunny in Philadelphia' have heard about Rob McElhenney's massive weight gain, which will be prominently featured in Season 7 (premieres Thurs., Sept. 15, 10PM ET on FX). You've probably even seen a few pictures of him looking quite a bit plumper. But, trust us -- you haven't seen him like this.

I visited the 'Sunny' set to get scoop on the new season -- including the upcoming high school reunion and an episode inspired by 'Toddlers & Tiaras' (more on that before the season premieres) -- and I got a very big surprise: Mac at his fattest point.

"We call him Fat Mac and he says, 'Well, will you call me Big Mac?' and I say, 'I'll call you Big Fat Mac,'" Glenn Howerton jokes. "But not just Big Mac, because Big Mac implies muscles and sort of strength."

And what does his loving wife Kaitlin Olson have to say about it? "People are gonna think you got fat and we had to write it in!" Yeah, we did kind of think that for a bit, but apparently this was all for comedy's sake.

Check out our video interviews to see Mac at his heaviest, and to hear about the side effects of his extreme weight gain, including a possibly not funny fatty liver and a completely destroyed metabolism.