Here's the MAL link!
Most magical girl series have a motif. Sailor Moon has space, Tokyo Mew Mew has animals, PMMM has pain and suffering... Well, this parody series has a weight loss motif.
First, some background. This manga is not a standalone title. It's actually a spinoff from a yaoi manga called Sakende Yaruze (also known as Shout Out Loud). Miracle Dieter Miyuki was a fictional anime in the universe of that manga, and eventually became its own manga. There are only two volumes -- more chapters were published in Japan, but were never collected into their own volume.
Has anyone seen episode seven of Wedding Peach? The one where the villains infuse some rice with "the essence of fatness" causing all the girls who eat it to get fat? That's essentially what the plot to this manga is. Yukimi, a chubby girl, transforms into the thin and beautiful Miyuki to fight the evil Chef Fiatee, who wants to make all the girls in the world fat for...some reason. We're never really told why Chef Fiatee is so fixated with teenage girls eating habits, but there you go.
To be honest, I went into this expecting a lot of fatshaming... And there was some, especially in the beginning. But it honestly surprised me by being less shame-y and more preachy. All it really did was promote healthy eating habits, like not skipping meals and eating your vegetables. Not too bad, all things considered, with a premise like this. Still, I'm not going to pretend that it didn't make me cringe every time Yukimi got picked on by the resident Mean Girl.