What do you do when you get a fat bird? We've given her a name.... little miss piggy. She seems to eat just as much as the others... and I have two Wyandottes and the other is quite small compared to her.
First of all check to make sure that it's fat, not fluid buildup of some kind due to an illness. Check her keelbone, the ridge on her chest between the wings. If it's fairly sharp and she's still fat, you may have a form of acitis (sp?).
If she is fat you can work at restricting her intake by feeding at regular times and removing the food after about a half an hour. You can do this twice a day. In addition, getting out and exercising is good. Some chickens, like people, will alleviate boredom by eating. Do they free range for part of the day?
In addition, examine the food they get for protein content. Some birds will eat more to get the protein they need and if they're getting a mix with lower protein, they'll be getting calories but not necessarily the ones they want. Your lean one will adjust their habits to the food as necessary.
You might also examine the treats she (they) get. When it was cold, I fed mine some extra corn in their scratch, but now that it's warming up, I'm removing it from their diet. They are fine on oats and wheat.