Inflation…hmm…
You know, as much as I love weight gain in stories, inflation to me has always seemed odd, both in real life, where people pump themselves up with air (I assume that’s the kind of inflation you mean) and in fiction, where the same thing happens, but usually in larger quantities. In real life stuff isn’t really of concern here though–because I write stuff in stories that I would never do in real life all the time, however I need to talk about it for a moment, to make clear why using it in stories doesn’t make sense to me.
Inflation is, at the end of the day, a temporary body modification, of a kind with pumping genitalia and temporary tattoos. It’s doing something to make your body look different, which can be more or less easily reversed, or which has no permanent effect (caveat, pumping for example can have permanent effects as far as growth goes, but the permanent effects are never as extreme as the temporary results).
This temporality, to me, always infects the MacGuffin in my mind. I hesitate to use inflation, because it always bears the implicit property of “reversibility” as a MacGuffin. Put simply, transformation by inflation, implies the existence of transformation by deflation, and the possibility that someone’s transformation can just be reversed tends to suck all of the tension out of the story.
Now, it can be that in the story world there actually is no means of reversing the change, and this can redeem the MacGuffin if used well, by creating an “unexpected consequence” for the character’s actions. However, I would rather just force someone to gain weight, than go through the longer, more complex motions of something like inflation. The simpler choice is “usually” the best.
Not always though–the one thing inflation can do that I find fascinating is yield a body type which fat can’t accomplish–that hard, taut round balloon gut which is so unnatural it becomes sexy. That can’t be easily accomplished without using this MacGuffin, but I don’t feel inclined towards that body type often in stories, so I have little use for it at the end of the day.
Long story short, I think inflation itself can be sexy, but don’t feel all that inclined towards it because it implies temporality. This sense of temporality, and the limited unique uses it has as a MacGuffin in the transformations it creates, leads me to generally neglect it in my stories–not because I dislike it, but because I can usually get the same result with less effort through a more standard weight gain trope.