Given how so many aspects are reliant on speed, movement, agility, etc. getting inflated or gaining lots of weight tends to put it quickly in the "debuff" catagory really quick. And if the fetish in question is the loss condition, then that's not going to be much fun imho. However there is a more fun approach if the protagonist was a feeder slaying and cooking monsters for a feedee and exploring the world for more complex, filling, and/or exotic dishes. Personally im into extreme sizes, and which alludes to a larger appetite - and therefore a progression of sorts of seeking out and hunting larger monsters for a better or more plentiful supply of food to keep them sated, but that's my two cents.