Since Bertha hated Sindy the most, she decided to get her revenge on her first. So Bertha made herself invisible then teleported to Sindy's house.
Bertha found Sindy in front of a full length mirror admiring her naked body.
That little tramp! Well she like to admire herself, so why don't I give her more to admire?
...
I'm perfect. Sindy thought to her self as she looked over her naked form. But little did she know that Bertha had made her unaware of the changes that were about to happen to her.
So Sindy began to expand. But Bertha did it slowly so that Sindy would have lots of chunky places. First Sindy began to develop large love handles. Then her beast began to expand.
"Oh no, my pretty. You don't get to expand there." Bertha said to herself. And suddenly Sindy's breasts stopped expanding and then shrunk down to half their previous size. Then the rest of Sindy's body continued to expand.
Next Sindy's calf plumped out. Then her butt suddenly shot out 2 feet behind her. "Careful there, you could put some ones eye out." Bertha said to herself. Sindy then continued to plump up till she was 700 pounds.
Her work finished, Bertha prepared to leave. But before she did, she made it so that every one thought that Sindy had always been fat, every one but Sindy of course.