Girls are fattened up upon making their contracts, for the purpose of being devoured.
Based on a fic I had recently commissioned for and am currently hosting on Fanfiction.net:
In this version of the Madokaverse, girls are appointed by Incubators to become a breed of Magical Girls known as Cibi (Latin for "food"). In exchange for one wish, they then become extremely fat. This type of fat consists of an extreme muscle growth with an outer layer of fat to cushion said muscles,is very firm and plush (rather than loose and flabby), and cannot be lost in any way. Its initial purpose is to strengthen them to fight flesh-eating monsters called Esurientes, but its true purpose is to make them more wholesome as food than regular people or animals.
Along with increased weight, Cibi also produce some kind of hormone that Esurientes need in order to reproduce, something which other beings are not capable of producing nearly as much, and which female humans are more suited for producing. When an Esuriens consumes enough of this hormone, it then bursts into spawn and creates the kind of energy that Incubators seek to combat entropy.
1) Characters are to be kept as consistent with their canon counterparts as actual events will allow for. No arbitrary changes to anyone's character, powers, wishes, where they live, etc.
For all canon outside of Madoka Magica, I have written a LiveJournal entry describing how different characters would be from their canon counterparts due to certain circumstances. (Caution: Contains spoilers for Oriko, Kazumi, Suzune, and Tart Magica, along with the mobage scenario The Hollow Little Mermaid.)
2) I reserve the right to modify anything you write as I see fit, for the sake of consistency, storytelling quality, or any other reason I deem fit.
Pics of each of the main five as they would appear as Cibi:
If anyone would like to see more, please donate DeviantArt points to the donation pool on my userpage there: