Of course, Jon couldn't see the extent of his "something interesting" wish. Yes, it caused Sarah to blow up into the fattest girl in school and inadvertently also turned her into his sister's girlfriend (odd, Jon never knew that Zoe was a lesbian), but that was only the beginning.
Already, another girl from school was growing fatter. It was Missy Parker, the girl that Sarah intimidated during Math class. And as she grew fatter, reality changed around her as though she were always the size that she currently was (eventually, though, she'd grow as big as Sarah), even as her own memories didn't change.
At the mall, Missy was shopping with her friends (there was no cheerleading practice today, so they headed straight to the mall after school). It was here that she started to notice that she was a bit chubbier than normal. But when she pointed it out to her friends, they acted like she was always that way. Why would they say that? It obviously wasn't true.