Don't worry, it happens a lot.
I could use my power for a lot of things, including a civil experiment:
I would target a population group (i.e. females at school) and give them the "ideal" body weight over the holidays. Then when school comes back, I'd see how long it takes for them to figure out I did it (I'd be prepared to wait). I'd also see reactions from several people (i.e. males at my school) which I could observe.
Why do it this way? Well, since I can't tell people, I may as well do something nobody will complain about. The "ideal" body weight would be
their ideal body weight, not society's.
I could also do it overnight, but that would be figured out to be supernatural immediately, eliminating thousands of possible explanations.