don't think such a spell exists. you could try a curse that would cause the person to get a 'taste of their own medicine' if that's what they've been making fun of the caster about something. but it won't be instant, it won't be obvious, and it won't be permanent. i think if a spell like that would exist, the person would gain like a dress size, they'd freak out, but then it would wear off in a month or so.
i think a karma hex or something small like that could work. it would cause the person to feel what the caster has been going though. [meaning get picked on, or the person would see themselves as fat] again, not permanent. for the person who is being picked on, a weight loss spell could help as a motivator, but it needs to be coupled with diet and exercise. a spell like the one you're looking for does not exist though because magick doesn't work instantly and cause such dramatic physical changes in a short period of time.