When people tell me i have fat chickens, i assume they're giving a compliment, like "look at those nice fat chickens." They don't know whether my chickens are fat. No one does unless they pick them up and feel their breast bone.
Someone here once gave me this guide.
Pick up your chicken and try to feel the breast bone.
If you can pinch the actual bone - not meat on the bone - with your full finger and thumb, you chicken is skinny.
If you find that your chicken has "cleavage"
she is fat.
If you find something in between, she's pretty good.
I feel really comfortable if i can just barely feel the breast bone of my chickens. When they're growing pullets and free ranging, they tend to be a little thinner than my ideal, but they're not unhealthy; they're just growing and well exercised.
I think that if you're feeding your chickens a normal chicken food, you're not likely to get fat chickens.
I hope this guide helps.