Fanedfox:
For me, it was a couple of cartoons, of course the weight standard, “ Pigs be Pigs” and another one with Porky Pig, a mouse and some monster guy. This cartoon ends with Porky and the mouse gorging on food showing huge bellies.
Same for me! "Bye Bye Bluebeard" was the name of that cartoon. I don't know how old I was when I first saw it, but it sure did affect me. The mouse lifting up the tablecloth to reveal his huge belly as he grinned a satisfied grin and patted and jiggled his belly excited me no end for some reason and sent me packing to my room to stuff pillows and blankets under my shirt because, at least in that moment, I wanted a big fat belly too. The same happened when I saw other, similar weight gain cartoons as a kid--Templeton gorging at the county fair in Charlotte's Web, a Popeye cartoon called "Egypt Us" in which Wimpy winds up eating all of the hamburgers that he, Popeye and Olive had planned to eat for a picnic, "Holiday for Drumsticks", a cartoon in which Daffy Duck is gets fat in time for Thanksgiving. Lots of them. Even one Gilligan's Planet cartoon in which Gilligan pads himself to look very fat. They all contributed to my padding experiences.
And like others in this thread have said, I think it was the thrill of being able to experience "being fat" immediately without the consequences and in private that was the attraction. That, and any excitement or sexual pleasure it brought.
I was pretty careful, but I nearly got caught be my parents a couple of times, which I suppose only added to the thrill of it.
I don't have any idea why it triggers some people and doesn't trigger others, only that it does, and did in my case. I watched a lot of cartoons. Weight gain in cartoons was usually seen as a gag, meant to make one laugh or smile, with the character usually eating to please or gratify themself, so maybe that had something to do with it, wanting to have a big full belly. I dunno. It's always been a mystery to me.