I think she´s yo-yo dieting the hell out of her body. She goes to extreme measures when a new music video is due, and gains it all back as soon as it´s over. She admittedly struggled with EDs when she was younger, and maybe she didn´t lay all the ED behaviour off (if she did recover at all, I don´t know)
I know a little about editing pictures, and it´s entirely possible to morph a chubby person´s body into a skinny minnie but the costs for editing a whole length video would be too high.
I´m not completely refusing the video editing theory; but for an extreme person like her it´s highly possible that she restricts like mad before every video.