Here’s a few off the top of my head.
- Feeder Fantasy by Fallboy has a male MC that gains weight. The original is in japanese but there is a english machine translation by humblsasquatch.
- Feeder Fantasy moded version that has a female MC that can gain weight and is already translated by StygainSkies and team.
Both of these versions the MC can get very large and are tied to player choices to make the MC gain weight. It also has some great art for the other females in game that can also gain.
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Olive And The Ruby Bra by frequenf has a female mc with weight gain I think this one is tied to story but I remember the MC gaining fast within the start of the story and can get large.
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Growing Port Lee (formerly Untitled Farming Game) by Chubberdy. I have not tried the new now named version but the original allowed you to gain based on what you ate. That I’m aware this is still the same in the updated version.