Well, Farouk was a big eater, which was the reason he made Karma so fat.
Come to think of it, maybe the reason no one else ever blimped up when possessed by the King is because he never possessed them long enough to indulge his appetites like he did with Karma (Has he ever possessed someone as long as he did Karma?)
Also, think Betsy would have become as fat as Mojo if she had stayed Mojofied?