I have 2 buff orps, and their normal look is pretty hefty looking. A lot of that is probabably feathers, though. If they are acting normal and active, then I wouldn't worry about it.
Laying hens are very good at regulating their own feed intake and rarely (I'd say never, but there's always a few outliers that would trip me up) get fat. Feel your chicken's chest--there is a bone sticking out there called the keel bone. If it's sticking out, your chicken is not overweight. If it sticks out a lot, your bird is underweight.
You should not restrict feed to laying hens--they should have all they can eat instead of being fed a set amount the way you'd feed a dog or a horse. Any disruptions in their feed can cause them to lay poorly.
Each breed has a weight. Look up how much the Buffs should weigh and then check Leghorns. Weigh ure birds and see if they are in the range. I think Buff hens shouldnt get over 6 pounds I THINK!