How about ruining someone’s life unreversibly by muscle growth?(Something like Onix’s BMOC maybe) And I want to ask you why you always making muscle growth by magic, since you said you don’t like wish-come-true tf stories. Couldn’t they do anything like workout?

You can ruin someone’s life through muscle growth, of course–but frankly I don’t find muscle at all attractive, and so, I just don’t have much interest in writing it. The reason for that, is that for me, ruination is all about forcing someone to give in to sloth and excess and gluttony, but maintaining that sort of muscle mass is all about regimentation, effort, and incredibly hard work. I can see the appeal in that too, of course, but personally it does little for me.

As far as change-by-magic and wish fulfillment go, your question is based on a misunderstanding. Wish fulfillment, as a type of story, doesn’t have anything to do with the type of MacGuffin being employed. For example, a character who wants to be a powerlifter, and is conventionally trained by a benevolent character to become a powerlifter, is still a wish fulfillment story–the character, in the end, got what they wanted with minimal conflict. 

In fact, the vast majority of my stories are magical in one way or another (or at the very least, not realistic–in the sense that a chance-by-science is just as impossible in real life as a change-by-magic would be) regardless of whether they contain weight gain or muscle growth, or whatever transformation is going on. I prefer magic because it allows stories to develop quicker, and for scenes to involve more contrast without requiring massive leaps in time. That’s a personal preference of mine, but of course you could have a story developed only through conventional means too.