Indeed "A Flaw in the Plan" is also the first thing that came to my mind, a short but well written story.
Otherwise there is also "Operation Adephagia" by Logitechk1 (although it is not his best story). A woman is assigned to join the harem of a criminal in order to retrieve information and compromising elements, but of course this criminal likes women with a healthy weight.
https://www.deviantart.com/logitechk1/art/Operation-Adephagia-Chapter-1-680740875