Delicious food causes trouble for Downton Costumers! While MTV wants to attract readers with made up controversy, it's not an invalid issue. Downton Abbey, like many period TV shows, borrows most of its wardrobe, often using actual period pieces. If an actor gains weight, there isn't time or fabric to remake the costume in a larger size, and there's only so much alterations can do, especially with all that beading! Not to mention, I expect there's a lot of rules about what can be done with the antique pieces.
Having done some films myself, I know how hard it is to keep the actors looking consistent, especially if you're filming chronological scenes months (or years!) apart. We ran into that several times over the course of making "The Shadow of the Bear", particularly with hairstyles/colors, although some costume pieces just plain wore out!
While Downton Abbey isn't small budget TV, it's still small budget compared to a film, where costume alterations can and do take place, with multiple versions often being made of each costume. So it's no wonder that the actors are being asked to cut down on the food... hey, I'll take some of those sandwiches off their hands! Do they have any gluten free ones? ;)