If you could create a size mechanic from a doujin inside a game, what would it be and how would it work?

I'm a big fan of level drain, but I can't see how it would make sense as a permanent/core mechanic without making the game loop super tedious. Maybe just as a thematic effect in a one-off game-over route?
I've seen SAO try it with the boys caught in the terrarium EXP farm, but I thought the fact that they have to grind against bugs was super lame, and other than the first loop it didn't do anything for me.