When it comes to the visual aspect of a weight gain game, one question can easily come to mind: how do you visualize the gain? Some games forego the visualization entirely and stick to letting your imagination do the work. Often these are either text-based games or pixel sprite games with only one set of character sprites. Where the weight gain is not explicitly shown, but implied.
Then you have one of the other prominent methods using things RPG Maker, where the overworld pixel sprites are edited to be larger. Sometimes this is coincided with a larger sprite to the side, showing the character in their full glory. To one side you have the actual gameplay, and on another is the player character, who gets larger as weight is gained. …Though this is at the cost of screen space.
So how do you like to see characters gain weight in games?