Cad:
I literally have a story where that exact thing happens to the character, even though immobility isn't my kink. (The character becomes a supervillain because he's secretly into immobility and he wants to be targeted by a superhero whose power is...well, she zaps you and you gain hundreds of pounds and you're too heavy and immobile to carry out the crime you're in the middle of doing. It's supposed to be a comedy, but you'll think it's sexy if you like immobility.)
My own favorite trope is when two feedee characters are also best friends/drinking buddies and they encourage each other platonically to order larger and larger portions of greasy bar food when they're hanging out together. Bonus points if things get a little steamy and sometimes they grab each other's love handles under the bar counter. BONUS bonus points if they're both so big that their fat squishes together when they try to carpool to and from the bar.
Wow, I really like bars. Can you tell I'm a total lush, or am I articulate enough to hide it?