Gaining

I really like the thought of mutual gaining, two people just encouraging each other to gorge themselves and admire their increasing girth.

Great way to describe this! 4F needs to be the new slang for fat & gaining people!

I've always been a fan of instant weight gain. Somebody gorges themself and slowly starts to grow. Seams stretch, buttons pop, their belly pushes the table farther away until they're huge and happy.