He's not fat. He's big shelled.
Seriously though. You should probably cut back on those cookies.
Back in 1990 there was a promotion (I guess) loosely based around Teenage Mutant Ninja Turtle Cookies.
Check out the commercial and home video below:
(Also note the fat Michaelangelo running at the end of the commercial. Classic!)
Blerg. I know that the cartoon theme song kicks ass and all... but listening to it on a loop like that would be a form of torture.